Semiconductor-oxides nanotubes-based composite particles useful for dye-removal and process thereof

ABSTRACT

Semiconductor-oxide nanotubes-based composite particles are useful for dye-removal. A method involves an ion-exchange mechanism operating under a dark-condition in an aqueous solution, for the processing of products consisting of the nanotubes of semiconductor-oxides deposited on, anchored to or attached to the surface of fly ash particles and metal-oxide (magnetic and non-magnetic) nanoparticles. The resulting micro-nano and nano-nano integrated composite particles can be used in the removal of an organic synthetic-dye from an aqueous solution and industry effluent via a surface-adsorption process, involving ion-exchange and electrostatic-attraction mechanisms. The composite particles can be recycled for the next cycle of dye-adsorption by decomposing the previously adsorbed dye on their surfaces via the use of either noble-metal-deposited or magnetically separable magnetic photocatalysts and exposure to ultraviolet (UV) or solar-radiation.

TECHNICAL FIELD OF THE INVENTION

The present invention primarily relates to semiconductor oxide nanotubes based composite particles useful for dye removal and process thereof. Particularly, the present invention relates to Semiconductor Oxide Nanotubes-Flyash and Semiconductor Oxide Nanotubes-Metal Oxide Composite Particles, their Preparation, method for removal of dye using these composite particles. More particularly, the present invention relates to a method for Recycling of these composite particles in the Dye-Removal Application.

BACKGROUND AND PRIOR ART OF THE INVENTION

Organic synthetic-dyes are extensively used in various industries such as the textile, leather tanning, paper production, food technology, agricultural research, light-harvesting arrays, photo-electrochemical cells, and hair-coloring. Due to their large-scale production, extensive use, and subsequent discharge of colored waste-waters, the toxic and non-biodegradable organic synthetic-dyes cause considerable environmental pollution and health-risk factors. Moreover, they also affect the sunlight penetration and oxygen solubility in the water-bodies, which in turn affect the under-water photosynthesis activity and life-sustainability. Moreover, due to their strong color even at lower concentrations, the organic synthetic-dyes generate serious aesthetic issues in the waste-water disposal. On the other hand, the toxic soluble heavy metal-cations cause serious problems to the ecosystem due to the serious health problems as a result of their accumulation in living tissues through the food-chain. Therefore, the removal of both highly stable organic synthetic-dyes and heavy metal-cations from the aqueous solutions and industry effluents is of a prime importance. Reference may be made to (V. K. Gupta, Suhas, “Application of Low-Cost Adsorbents for Dye Removal—A Review”, Journal of Environmental Management 2009, 90, 2313-2342) wherein, the investigations conducted in the past using the different adsorbents such as orange peel, rice husk, coconut shell, carbon black, zeolites, carbon nanotubes, and flyash have been summarized. Typically, flyash (solid and hollow, also known as cenospheres) which is a waste by-product of thermal power plants, comprising the mixture of different metal-oxides such as silica (SiO₂, 50-85 wt. %), alumina (Al₂O₃, 5-20 wt. %), iron oxide (Fe₂O₃, 5-15 wt. %), and trace amount of oxides of other elements such as calcium, titanium, magnesium, and toxic heavy metals such as arsenic, lead, and cobalt, has been environmentally hazardous and pose major disposal and recycling problems worldwide. Reference may be made to (H. Liu, “Method to Produce Durable Non-Vitrified Fly Ash Bricks and Blocks”, U.S. Pat. No. 7,998,268; B. R. Reddy, K. M. Ravi, “Methods of Formulating a Cement Composition”, U.S. Pat. No. 7,913,757; R. L. Hill, C. R. Jolicoeur, R. Carmel, M. Page, I. Spiratos, T. C. To, “Sacrificial Agents for Fly Ash Concrete”, U.S. Pat. No. 7,901,505) wherein, flyash has been traditionally used for landfill, manufacturing constructional materials such as cement, concrete, and bricks. Reference may be made to (K. Vasanth Kumar, V. Ramamurthi, S. Srinivasan, “Modeling the Mechanism Involved During the Sorption of Methylene Blue onto Flyash”, Journal of Colloid and Interface Science 2005, 284, 14-21; M. Matheswaran, T. Karunanithi, “Adsorption of Chrysiodine R by using Fly Ash in Batch Process”; Journal of Hazardous Materials 2007, 145, 154-161; D. Mohan, K. P. Singh, G. Singh, K. Kumar, “Removal of Dyes from Wastewater Using Flyash a Low-Cost Adsorbent”, Industrial and Engineering Chemistry Research 2002, 41, 3688-3695; S. Kara, C. Aydiner, E. Demirbas, M. Kobya, N. Dizge, “Modeling the Effects of Adsorbent Dose and Particle Size on the Adsorption of Reactive Textile Dyes by Fly Ash”, Desalination 2007, 212, 282-293) wherein, new industrial applications of flyash such as conductive/non-conductive filler in polymers, binder for agglomerating reactive mine tailings, manufacturing zeolites and microfiltration membranes, and adsorption of oil from aqueous solutions have been demonstrated. Flyash has also been utilized for the adsorption of different organic synthetic-dyes including remazol red RB 133, remazol blue, rifacion yellow HED, chrysoidine R, crystal violet, Rhodamine B, C.I. reactive black, 2-picoline, and acid red (AR1) from the aqueous solutions. Reference may be made to (M. Visa, C. Bogatu, A. Duta, “Simultaneous Adsorption of Dyes and Heavy Metals from Multicomponent Solutions using Fly Ash”, Applied Surface Science 2010, 256, 5486-5491; S. Wang, M. Soudi, L. Li, Z. H. Zhu, “Coal Ash Conversion into Effective Adsorbents for Removal of Metals and Dyes from Wastewater”, Journal of Hazardous Materials 2006, B133, 243-252; K. Ojha, N. C. Pradhan, A. N. Samanta, “Zeolite from Fly Ash: Synthesis and Characterization”, Bulletin of Materials Science 2004, 27, 555-563) wherein, flyash has been utilized for the adsorption of heavy metal-cations such as Sn²⁺/Sn⁴⁺, Fe²⁺/Fe³⁺, Pb²⁺, Zn²⁺, Cu²⁺, Mn²⁺, Ti⁴⁺, and others from aqueous solutions. The major advantage of using flyash for these applications is that it can be separated from the treated aqueous solutions via gravity settling. However, the major drawbacks of prior art-1 are as follows.

-   -   (1) Flyash has very low specific surface-area, and as a result,         exhibits very low capacity for adsorbing organic synthetic-dyes         and heavy metal-cations on its surface.     -   (2) Novel techniques to increase the specific surface-area of         flyash, without affecting its typical spherical morphology and         yet increasing its capacity for adsorbing organic synthetic-dyes         and heavy metal-cations, are unknown.     -   (3) Adsorption of organic synthetic-dyes and heavy metal-cations         on the surface of flyash generates large amount of sludge which         creates further handling, disposal, and recycling issues which         are at present not addressed satisfactorily.     -   (4) Novel value-added products based on the innovative         surface-modifications of flyash for the potential applications,         typically the removal of organic synthetic-dyes from the aqueous         solutions, are currently lacking.     -   (5) Novel approaches for recycling the flyash as a catalyst in         the dye-removal application, by decomposing the previously         adsorbed-dye from its surface, are currently lacking.

Hence, it is vital to develop innovative approaches to enhance the specific surface-area of flyash to increase its capacity for the adsorption of organic synthetic-dyes and heavy metal-cations. Innovative approaches are also needed to be developed to decompose the previously adsorbed-dye from the surface of flyash to recycle it for the next-cycles of dye-adsorption as a catalyst Reference may be made to (S. Shukla, S. Seal, J. Akesson, R. Oder, R. Carter, K. Scammon, “Study of Mechanism of Electroless Copper Coating of Flyash Cenosphere Particles”, Applied Surface Science 2001, 181, 35-50; S. Shukla, S. Seal, Z. Rahaman, K. Scammon, “Electroless Copper Coating of Cenospheres using Silver Nitrate Activator”, Materials Letters 2002, 57, 151-156; S. Shukla, K. G. K. Warder, K. V. Baiju, T. Shijitha, “Novel Surface-Modifications for Flyash and Industrial Applications Thereof”, U.S. patent application Ser. No. 13/612,363 (Filed on 12 Sep. 2012), PCT Application No. PCT/IN2010/000735 (Filed on 11 Nov. 2010)) wherein, as far as flyash particles with the surface-adsorbed heavy metal-cations are concerned, the Sn²⁺ cations adsorbed on the surface of flyash particles have been reported to be useful as sensitizer in an electroless metal (Cu/Ag)-coating of flyash particles using the Sn—Pd catalyst system. However, the major drawbacks of prior art-2 are as follows.

-   -   (6) Flyash particles with the surface-adsorbed Sn²⁺ cations do         not find other novel potential industrial applications.     -   (7) Flyash particles with the surface-adsorbed metal-cations,         other than Sn²⁺, are not suitable for the electroless         metal-coating application.     -   (8) Flyash particles with the surface-adsorbed metal-cations,         other than Sn²⁺, have not been utilized for other potential         applications.

Hence, the new potential industrial applications are required to be invented for improving the handling, disposal, and recycling issues of flyash with the surface-adsorbed heavy metal-cations. Reference may be made to (T. Kasuga, H. Masayoshi, “Crystalline Titania and Process for Producing the Same”, U.S. Pat. No. 6,027,775; T. Kasuga, H. Masayoshi, “Crystalline Titania having Nanotube Crystal Shape and Process for Producing the Same”; U.S. Pat. No. 6,537,517; N. Harsha, K. R. Ranya, S. Shukla, S. Biju, M. L. P. Reddy, K. G. K. Warder; “Effect of Silver and Palladium on Dye-Removal Characteristics of Anatase-Titania Nanotubes”, Journal of Nanoscience and Nanotechnology 2011, 11, 2440-2449; N. Harsha, K. R. Ranya, K. B. Babitha, S. Shukla, S. Biju, M. L. P. Reddy, K. G. K. Warder, Hydrothermal Processing of Hydrogen Titanate/Anatase-Titania Nanotubes and Their Application as Strong Dye-Adsorbents”, Journal of Nanoscience and Nanotechnology 2011, 11, 1175-1187; P. Hareesh, K. B. Babitha, S. Shukla, “Processing Fly Ash Stabilized Hydrogen Titanate Nano-Sheets for Industrial Dye-Removal Application”, Journal of Hazardous Materials 2012, 229-230, 177-182) wherein, the removal of heavy metal-cations and organic synthetic-dyes from the aqueous solutions via the surface-adsorption process, involving the ion-exchange and electrostatic-attraction mechanisms operating in the dark-condition, using the hydrothermally processed nanotubes of semiconductor-oxides such as the hydrogen titanate (H₂Ti₃O₇, HTN) and anatase-titania (TiO₂, ATN) have been demonstrated. The HTN and ATN possess very high specific surface-area typically about 100-200 times that of as-received flyash particles. Hence, the adsorption-capacity of HTN and ATN for adsorbing organic synthetic-dyes and heavy metal-cations is extremely large. However, the major drawbacks of prior art-3 are as follows.

-   -   (9) HTN and ATN cannot be separated quickly from the treated         aqueous solution via gravity settling.     -   (10) HTN and ATN are non-magnetic; hence, they cannot be         separated from the treated aqueous solution using an external         magnetic field.

In view of the prior arts 1-3 and their limitations, it appears that there is a need for the development of novel composite materials which would exhibit higher capacities for surface-adsorbing the organic synthetic-dyes and heavy metal-cations and can be separated quickly from the treated aqueous solution via gravity-settling or using an external magnetic field. Flyash particles have lower dye-adsorption capacity but can be separated from the treated aqueous solution via the gravity settling. On the other hand, the nanotubes of semiconductor-oxides have higher dye-adsorption capacity but cannot be separated from the treated aqueous solution via the gravity settling. Hence, this suggests that a micro-nano composite material consisting of the nanotubes of semiconductor-oxides, such as the hydrothermally processed HTN and ATN, deposited on the surface of flyash particles can serve the purpose. Reference may be made to (S. Shukla, K. G. K. Waffler, M. R. Varma, M. T. Lajina, N. Harsha, C. P. Reshmi, “Magnetic Dye-Adsorbent Catalyst”, U.S. patent application Ser. No. 13/521,641 (Filed on 11 Jul. 2012), PCT Application No. PCT/IN2010/000198 (Filed on 29 Mar. 2010); L. Thazhe, A. Shereef, S. Shukla, R. Pattelath, M. R. Varma, K. G. Suresh, K. Patil, K. G. K. Warrier, “Magnetic Dye-Adsorbent Catalyst: Processing, Characterization, and Application”, Journal of American Ceramic Society 2010, 93(11), 3642-3650) wherein, the magnetic dye-adsorbent catalyst, consisting of the “core-shell” nanocomposite particles with the core of a magnetic ceramic particle and the shell of nanotubes of semiconductor-oxide, has been developed via the hydrothermal treatment of magnetic photocatalyst (processed via the Stober and sol-gel methods) followed by typical washing-cycles, to facilitate the quick settling of HTN and ATN using an external magnetic field. Reference may be made to (C. C. Sheng, L. T. Gui, C. X. Hua, L. L. Wu, L. Q. Cheng, XQing, N. Z. Wu, “Preparation and Magnetic Property of Multi-Walled Carbon Nanotube/α-Fe₂O₃ Composites”, Transactions of Nonferrous Metals Society of China, 2009, 19, 1567-1571; E. Santala, M. Kemell, M. Leskela, M. Ritala, “The Preparation of Reusable Magnetic and Photocatalytic Composite Nanofibers by Electrospinning and Atomic Layer Deposition”, Nanotechnology 2009, 20, 035602; S. K Mohapatra, S. Banerjee, M. Misra, “Synthesis of Fe₂O₃/TiO₂ Nanorod-Nanotube Arrays by Filling TiO₂ Nanotubes with Fe”, Nanotechnology 2008, 19 315601, Fei Liu, Yinji Jin, Hanbin Liao, Li Cai, Meiping Tong, Yanglong Hou, “Facile Self-Assembly Synthesis of Titanate/Fe₃O₄ Nanocomposites for the Efficient Removal of Pb²⁺ from Aqueous Systems”, Journal of Physical Chemistry A, DOI: 10.1039/c2ta00099g) wherein, the magnetic nanocomposites having the morphology other than the “core-shell” morphology, consisting of magnetic nanoceramic particles deposited on the surface of TiO₂ nanotubes, have also been processed via different techniques including the precipitation-calcination, electrospinning-atomic layer deposition, pulsed electrodeposition, and self-assembly process. However, the major drawbacks of prior art-4 are as follows.

-   -   (11) The combination of sol-gel and hydrothermal methods is not         applicable to the flyash particles since SiO₂, a major         constituent of flyash particles, is soluble in a highly alkaline         aqueous solution involved in the hydrothermal treatment.     -   (12) Innovative techniques for depositing the nanotubes of         semiconductor-oxides on the surface of flyash are not known.     -   (13) Other techniques including the precipitation-calcination,         electrospinning-atomic layer deposition, and pulsed         electrodeposition are not suitable for depositing the nanotubes         of semiconductor-oxides on the surface of flyash.     -   (14) The self-assembly process produces the magnetic         nanocomposite with the magnetic nanoparticles uniformly         dispersed on the surface of semiconductor-oxides nanotubes which         reduces the potential sites on the surface of nanotubes required         for the adsorption of dye molecules and the metal-cations from         the aqueous solutions. Moreover, the self-assembly method also         requires the use of an acid for obtaining the said morphology.     -   (15) Innovative techniques for attaching or anchoring the HTN or         ATN to the surface of magnetic nanoparticles, typically at their         short-edges (tube-openings), are not currently available. As a         result, the said product cannot be synthesized using any of the         existing processes.

As a consequence, there is an urgent need to develop novel methods for depositing the nanotubes of semiconductor-oxides on the surface of flyash particles. Since the flyash particles are non-magnetic, they cannot be separated from an aqueous solution using an external magnetic field. Hence, it is also essential to demonstrate the deposition of the nanotubes of semiconductor-oxides on the surface of magnetic metal-oxide nanoparticles (instead of flyash) by attaching or anchoring them to the magnetic particle-surface using the same innovative mechanism which is employed in the case of flyash particles. Thus, the novel composite materials consisting of the nanotubes of semiconductor-oxides deposited on the surface of both the non-magnetic flyash and attached to (or anchored to) the surface of magnetic metal-oxide nanoparticles, via an innovative approach, would provide new ways of efficiently treating the aqueous solutions containing the harmful organic synthetic-dyes and heavy metal-cations. It would also provide new ways for the separation and recycling the flyash, without and with the surface-adsorbed metal-cations, as value-added products for the dye-removal application. As mentioned above, flyash (without and with the surface-adsorbed metal-cations), HTN, ATN, and magnetic composites can be used as dye-adsorbents. In order to recycle these dye-adsorbents as catalysts for the next-cycles of dye-adsorption, it is necessary to remove the previously-adsorbed dye from their surfaces. Reference may be made to (Z. Geng, Y. Lin, X. Yu, Q. Shen, L. Ma, Z. Li, N. Pan, X. Wang, “Highly Efficient Dye Adsorption and Removal: A Functional Hybrid of Reduced Graphene Oxide-Fe₃O₄ Nanoparticles as an Easily Regenerative Adsorbent”, Journal of Materials Chemistry 2012, 22, 3527-3535; M. Visa, L. Andronic, D. Lucaci, A. Duta, “Concurrent Dyes Adsorption and Photo-Degradation on Fly Ash Based Substrates”, Adsorption 2011, 17, 101-108; J.-T. Li, B. Bai, Y-L. Song, “Degradation of Acid Orange 3 in Aqueous Solution by Combination of Fly Ash/H₂O₂ and Ultrasound Irradiation”, Indian Journal of Chemical Technology 2010, 17, 198-203) wherein, annealing under the moderate conditions (at 400° C. for 1 h) for removing the previously adsorbed Rhodamine B dye from the surface of reduced graphene oxide-Fe₃O₄ composite has been reported. The mechanical mixture of flyash and TiO₂ powders has been employed for the decomposition of organic synthetic-dye on the surface of flyash under the ultraviolet (UV)-radiation exposure. The recycling of flyash via the simultaneous dye-adsorption on its surface and dye-degradation using the combination of H₂O₂ and ultrasound-irradiation (Fenton-like reaction) has been reported. Reference may be made to (S. Shukla, K. G. K. Warrier, K. B. Babitha, “Methods for Decomposition of Organic Synthetic-Dyes using Semiconductor-Oxides Nanotubes via Dark-Catalysis”, PCT Application No. PCT/IN2013/000319 (Filed on 17-May-2013), Indian Patent Application 2555DEL2012 (Filed on 17 Aug. 2012)) wherein, the combination of hydrothermally processed HTN or ATN and H₂O₂ has been used to degrade the previously adsorbed organic synthetic-dye in an aqueous solution, typically in the dark-condition, without the use of external-irradiation and external power-source. In such case, the dye-decomposition is achieved through the generation and attack of both the free hydroxyl-radicals (OH⁻) and superoxide-ions (O₂ ⁻) which are generated by the HTN and ATN in the presence of H₂O₂. Reference may be made to (M. S. Yalfani, S. Contreras, F. Medina, J. Sueiras, “Direct Generation of Hydrogen Peroxide from Formic Acid and O₂ using Heterogeneous Pd/α-Al₂O₃ Catalysts”, Chemical Communications, 2008, 3885-3887) wherein, the Pd-deposited on alumina (Al₂O₃) substrate has been utilized to generate H₂O₂ in-situ using the formic acid (HCOOH) and dissolved oxygen (O₂) which can be utilized for the decomposition of organic synthetic-dyes in an aqueous solution via the Fenton/Fenton-like reactions. However, the major drawbacks of prior-art 5 are as follows.

-   -   (16) Innovative techniques for removing the previously         adsorbed-dye from the surface of various adsorbents mentioned         above, for recycling them as catalysts, are unknown.     -   (17) The annealing treatment, conducted even under the moderate         conditions, can destroy the nanotube-morphology and the phase         structure;     -   (18) Flyash and TiO₂ cannot be separated, after the         dye-decomposition, from their mechanical mixture since both are         non-magnetic; hence, the recycling of flyash is not possible         from the mechanical mixture of flyash and TiO₂ powders for         reusing the former separately for the dye-adsorption process         conducted in the dark-condition.     -   (19) The use of a magnetic photocatalyst, consisting of the         “core-shell” nanocomposite particles with the core of a magnetic         ceramic particle, an intermediate insulating layer of silica         (SiO₂) or an organic polymer, and the shell of nanocrystalline         particles of semiconductor-oxide photocatalyst such as         anatase-TiO₂, has never been reported for the recycling of         dye-adsorbents typically the flyash without and with the         surface-modifications.     -   (20) The adsorbents such as the HTN and ATN cannot be recycled         via the mechanically mixing and UV-exposure method involving the         use of TiO₂ photocatalyst since the nanocrystalline TiO₂         particles have a tendency to get attached to the HTN and ATN via         an ion-exchange mechanism (which has been disclosed here) which         reduces the total number of potential-sites available for the         dye-adsorption for a given amount of dye-adsorbent. The latter         issue becomes severe with the increasing number of         dye-adsorption cycles.     -   (21) The recycling of flyash particles using the combination of         H₂O₂ (Fenton-like reactions) and ultrasound-irradiation is a         costlier process.     -   (22) The efficient methods to recycle the flyash, in the         dye-removal application, without the use of external-power         source are not known.

Hence, it is essential to develop simpler, easier, cost-effective, efficient, and innovative processes to remove or decompose the previously adsorbed-dye from the surface of dye-adsorbents, typically the flyash without and with the surface-modifications, to make their recycling possible in the dye-removal application.

OBJECTIVE OF THE INVENTION

The main objective of the present invention is to provide semiconductor-oxides nanotubes-based composite particles useful for dye-removal and process thereof.

Another objective of the present invention is to provide semiconductor-oxide nanotubes-based composite particles typically related to the potential application concerning the removal of organic synthetic-dyes from the aqueous solutions via the surface-adsorption process, involving the ion-exchange and electrostatic-attraction mechanisms, operating in the dark-condition.

Still another objective of the present invention is to provide the processes for the preparation of semiconductor-oxide nanotubes-flyash and semiconductor-oxide nanotubes-metal oxide composite particles.

Yet another objective of the present invention is to provide innovative products, and processes for the recycling of flyash, with the surface-adsorbed metal-cations, as a value-added waste by-product.

Yet another objective of the present invention is to provide a method involving both the surface-sensitization of flyash particles (by adsorbing the cations (either metal-ions (M^(n+)) or protons (H⁺)) and an ion-exchange mechanism operating under the dark-condition in an aqueous solution for the processing of novel semiconductor-oxide nanotubes-flyash composite particles.

Yet another objective of the present invention is to provide the optimum parameters for the processing of semiconductor-oxide nanotubes-flyash composite particles via a method involving both the surface-sensitization of flyash particles and an ion-exchange mechanism operating under the dark-condition in an aqueous solution.

Yet another objective of the present invention is to provide the doped-nanotubes of semiconductor-oxides to increase the dye-adsorption capacity of semiconductor-oxide nanotubes-flyash composite particles for the dye-removal application.

Yet another objective of the present invention is to provide a process for the preparation of semiconductor-oxide nanotubes-metal oxide nanocomposite particles which does not involve the surface-sensitization step.

Yet another objective of the present invention is to provide a method for decomposing the previously adsorbed-dye on the surface of as-received flyash particles or the semiconductor-oxide nanotubes-flyash composite particles (non-magnetic), by using the combination of a magnetic photocatalyst and an exposure to the UV or solar-radiation to recycle the as-received flyash particles or the semiconductor-oxide nanotubes-flyash composite particles as catalyst for the repeated cycles of dye-adsorption conducted in the dark-condition.

Yet another objective of the present invention is to provide a method for decomposing the previously adsorbed-dye on the surface of semiconductor-oxide nanotubes-metal oxide magnetic nanocomposite particles, by using the combination of noble-metal-deposited nanocrystalline anatase-TiO₂ photocatalyst (non-magnetic) and an exposure to the UV or solar-radiation, to recycle the magnetic nanocomposite as a catalyst for the repeated cycles of dye-adsorption conducted in the dark-condition.

Yet another objective of the present invention is to provide the semiconductor-oxide nanotubes-flyash composite particles which are magnetic in nature.

Yet another objective of the present invention is to provide a process for the preparation of semiconductor-oxide nanotubes-flyash composite particles which are magnetic in nature.

Yet another objective of the present invention is to provide the semiconductor-oxide nanotubes-flyash composite particles which are magnetic in nature for the dye-removal application.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides a composite particle which comprises,

-   -   (i) metal-cations (M^(n+)) or protons-sensitized flyash particle         of diameter in the range of 0.3-100 μm,     -   (ii) nanotubes of semiconductor-oxides deposited on the surface         of metal-cations or protons-sensitized flyash particle.

In one embodiment of the present invention the nanotubes are in the range of 1-15 wt. %, the meal-cations (M^(n+)) or protons in the range of 1-80 wt. %, and the balance being the weight of flyash particles.

In an embodiment of the present invention the flyash particle consists of the mixture of silica (SiO₂, 50-85 wt. %), alumina (Al₂O₃, 5-20 wt. %), iron oxide (Fe₂O₃, 5-15 wt. %), and trace amount of oxides of other elements selected from calcium, titanium, magnesium, and toxic heavy metals selected from arsenic, lead, and cobalt.

In another embodiment of the present invention the metal-cations (M^(n+)) are selected from the group consisting of Sn²⁺/Sn⁴⁺, Fe²⁺/Fe³⁺, Pb²⁺, Zn²⁺, Cu²⁺, Mn²⁺, and Tr.

Still in another embodiment of the present invention the nanotubes of semiconductor-oxides are selected from the group consisting of hydrothermally processed hydrogen titanate (HTN, H₂Ti₃O₇ or the lepidocrocite-type), anatase-TiO₂ (ATN), and Ag-doped ATN.

Still in another embodiment of the present invention the nanotubes of semiconductor-oxides are optionally attached to the magnetic metal-oxide nanoparticles at the short-edges (tube-openings) and/or surface-deposited with the magnetic metal-oxide nanoparticles which are selected from the group consisting of γ-Fe₂O₃, Fe₃O₄, CoFe₂O₄, NiFe₂O₄, MnFe₂O₄, Co, Fe, and Ni.

Still in another embodiment of the present invention a process for the preparation of a composite particle, wherein the said process comprises the steps of,

-   -   (a) dissolving a metal-salt (1-70 g·l⁻¹) in water having initial         solution-pH-1-2 adjusted using the 0.1-1 M HCl solution under         continuous stirring for 30-300 min at temperature ranging         between 20-30° C., followed by suspending flyash particles (1-25         g·l⁻¹) into it under continuous stirring for 30-300 min at         temperature ranging between 20-30° C. to obtain the suspension         of metal-cations (M^(n+)) sensitized flyash particles;     -   (b) optionally suspending flyash particles (1-25 g·l⁻¹) in         50-250 ml of water having the initial solution-pH-1-2 adjusted         using the 0.1-1 M HCl solution under continuous stirring for         30-300 min at temperature ranging between 20-30° C. to obtain         the suspension of protons-sensitized flyash particles;     -   (c) adding hydrothermally processed nanotubes of         semiconductor-oxides (0.1-5 g·l⁻¹) to the suspension of cations         (M^(n+)) or protons-sensitized flyash particles as obtained in         step (a) or in step (b), followed by sonicating the suspension         for 5-30 min;     -   (d) stirring the suspension obtained in step (c) continuously         for 30-300 min at temperature ranging between 20-30° C. to         obtain the composite of semiconductor-oxide nanotubes and         cations (M^(n+)) or protons-sensitized flyash particles;     -   (e) separating the composite particles as obtained in step (d)         using a centrifuge operated at 2000-4000 rpm or an external         magnetic field, followed by washing the composite particles         using water for 30 min-2 h multiple-times till the pH of         filtrate remains unchanged or neutral, followed by separation         and drying the composite particles in an oven at 70-90° C. for         10-15 h.

Still in another embodiment of the present invention the metal-salt used in step (a) is selected from the group consisting of chloride, nitrate, sulfate-salts of Sn²⁺/Sn⁴⁺, Fe²⁺/Fe³⁺, Pb²⁺, Zn²⁺, Cu²⁺, Mn²⁺, Ti⁴⁺, and others.

Still in another embodiment of the present invention a nanocomposite particle which comprises,

-   -   (i) metal-oxide nanoparticles in the range of 5-70 wt. %;     -   (ii) nanotubes of semiconductor-oxides in the range of 30-95 wt.         % attached to the surface of metal-oxide nanoparticles at the         short-edges (tube-openings).

Still in another embodiment of the present invention the metal-oxide nanoparticles are selected from the group consisting of γ-Fe₂O₃ (magnetic), Fe₃O₄ (magnetic), SnO/SnO₂, PbO, ZnO, CuO, MnO, and TiO₂.

Still in another embodiment of the present invention the nanotubes of semiconductor-oxides are selected from the group consisting of hydrothermally processed hydrogen titanate (HTN, H₂Ti₃O₇ or the lepidocrocite-type) and anatase-TiO₂ (ATN).

Still in another embodiment of the present invention a process for the preparation of a nanocomposite particle, wherein the said process comprises the steps of,

-   -   (A) Dispersing the 0.5-10 g·l⁻¹ of metal-oxide nanoparticles in         water having the neutral solution-pH (˜6.5-7.5) under continuous         stirring for 5-30 min at temperature in the range of 20-30° C.;     -   (B) adding 0.5-10 g·l⁻¹ of hydrothermally processed nanotubes of         semiconductor-oxides in the suspension obtained in step (A)         under continuous stirring for 5-30 min at temperature in the         range of 20-30° C., followed by sonicating the suspension for         5-30 min, subsequently stirring the suspension continuously for         1-10 h in the dark-condition to obtain the semiconductor-oxide         nanotubes-metal oxide nanocomposite particles;     -   (C) separating the semiconductor-oxide nanotubes-metal oxide         nanocomposite particles using a centrifuge operated at 2000-4000         rpm or an external magnetic field, followed by washing the         nanocomposite particles using water for 30 min-2 h till the pH         of filtrate remains unchanged or neutral, followed by separation         and drying the nanocomposite particles in an oven at 70-90° C.         for 10-15 h.

Still in another embodiment of the present invention The composite and nanocomposite particles are useful for the application involving the dye-removal from an aqueous solution and industry-effluent via the surface-adsorption mechanism operating in the dark-condition.

Still in another embodiment of the present invention a process for the surface-cleaning and the recycling of composite/nanocomposite particles after the adsorption of an organic synthetic-dye from an aqueous solution via the surface-adsorption mechanism operating in the dark-condition, comprising the steps of,

-   -   a. suspending 1-30 g·l⁻¹ of composite/nanocomposite particles         having the surface-adsorbed organic synthetic-dye (0.1-3 mg·g⁻¹)         in water under continuous stirring for a period ranging between         5-30 min at temperature ranging between 20-30° C.;     -   b. suspending a photocatalyst (10-60 wt. % of total weight of         suspended solid particles) selected from the group of         nanocrystalline anatase-TiO₂-coated SiO₂/γ-Fe₂O₃ magnetic         photocatalyst or the noble-metal-deposited nanocrystalline         anatase-TiO₂ in the suspension obtained in step (a) under         continuous stirring, followed by sonicating the suspension for         5-30 min, subsequently stirring the suspension continuously         under the UV or solar-radiation exposure for 1-10 h;     -   c. centrifuging the solution at 2000-4000 rpm to separate the         composite/nanocomposite particles and photocatalyst together,         followed by washing using water for 30 min-2 h multiple-times         till the pH of filtrate remains unchanged or neutral;     -   d. separating the photocatalyst particles from the         surface-cleaned composite/nanocomposite particles using an         external magnetic field;     -   e. drying both the photocatalyst particles and         composite/nanocomposite particles in an oven at 70-90° C. for         10-15 h for the reuse.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 represents the TEM images of as-received flyash particles (a) and hydrothermally processed HTN (b).

FIG. 2 represents the XRD patterns of as-received flyash particles (a) and hydrothermally processed HTN (b).

FIG. 3 represents the EDX spectra of as-received flyash particles (a) and hydrothermally processed HTN (b).

FIG. 4 represents the high magnification TEM images of as-received flyash particle (a) and the micro-nano integrated HTN-flyash composite particle (b) showing the surface and interphase regions. In (b), the composite particle is processed via an ion-exchange mechanism operating under the dark-condition in an aqueous solution using the Sn²⁺ cations for the surface-sensitization of as-received flyash particles.

FIG. 5 represents the XRD pattern (a) and EDX spectrum (b) of micro-nano integrated HTN-flyash composite particles processed via an ion-exchange mechanism, operating under the dark-condition in an aqueous solution, using the Sn²⁺ cations for the surface-sensitization of as-received flyash particles. The initial SnCl₂ concentration is 40 g·l⁻¹.

FIG. 6 represents the variation in the normalized concentration of surface-adsorbed MB dye as a function of contact time for the different dissolution time of SnCl₂ which is varied as 30 min (a), 2 h (b), and 4 h (c). The initial SnCl₂ concentration is varied as 5 (i), 20 (ii), 40 (iii), and 60 g·l⁻¹ (iv). The initial solution-pH during the dye-adsorption measurement is ˜7.5.

FIG. 7 represents the variation in q_(e) as a function of initial SnCl₂ concentration obtained for the different dissolution time of SnCl₂ which is varied as 30 min (a), 2 h (b), and 4 h (c). The initial solution-pH during the dye-adsorption measurement is ˜7.5. The maximum value of q_(e) is indicated by an arrow.

FIG. 8 represents the variation in the normalized concentration of surface-adsorbed MB dye as a function of contact time obtained for the HTN-flyash composite particles (non-magnetic) processed using the different contact time of SnCl₂ solution with the as-received flyash particles (a) and that of the hydrothermally processed HTN with the Sn²⁺-sensitized flyash particles (b). (i)-(iv) represent the contact time of 1-4 h at the interval of 1 h. The initial SnCl₂ concentration and its dissolution time are 20 g·l⁻¹ and 2 h. In (a), the contact time HTN and Sn²⁺-activated flyash particles is 4 h. In (b), the contact time SnCl₂ solution with the suspended as-received flyash particles is 4 h.

FIG. 9 represents (a) the variation in the normalized concentration of surface-adsorbed MB dye as a function of contact time obtained for the different initial MB dye concentrations which is varied as 15 (i), 30 (ii), 60 (iii), and 90 μM (iv) measured at the initial solution-pH of ˜7.5. (b) represents calculated variation in q_(e), as a function of initial MB dye concentration obtained for the HTN-flyash composite.

FIG. 10 represents the TEM image (a) and XRD pattern (b) of hydrothermally processed ATN. The precursor used for the hydrothermal processing of ATN is the nanocrystalline anatase-TiO₂ particles processed via acetic-acid (catalyst) modified sol-gel method. In (a), the inset shows the corresponding SAED pattern. In (b), A represents the anatase-TiO₂.

FIG. 11 represents the variation in q_(e) as a function of initial MB dye concentration as obtained for the hydrothermally processed Ag-doped ATN (a) and the Ag-doped ATN-flyash composite particles (b) having different values of Ag/Ti mole-ratio—0 (i), 1 (ii), and 5% (iii).

FIG. 12 represents the TEM image (a), EDX spectrum (b), and XRD pattern (c) obtained using the conventional anatase-TiO₂-coated (cycle-5) SiO₂/γ-Fe₂O₃ magnetic photocatalyst (containing 26 wt. % TiO₂, 25 wt. % γ-Fe₂O₃, and 49 wt. % SiO₂) processed using the combination of modified-Stober and conventional sol-gel methods. A and M represent the anatase-TiO₂ and γ-Fe₂O₃ (maghemite) structures.

FIG. 13 represents the effect of a moderate external magnetic field on the magnetic separation of anatase-TiO₂-coated (cycle-5) SiO₂/γ-Fe₂O₃ magnetic photocatalyst (containing 26 wt. % TiO₂, 25 wt. % γ-Fe₂O₃, and 49 wt. % SiO₂) from an aqueous solution. (a) and (b) show the digital photographs taken after the addition of magnetic photocatalyst to an aqueous solution without and with the presence of a moderate external magnetic field (holding time of 5 min). In (b), the arrow at the right-side bottom-corner shows the position of an external magnet.

FIG. 14 represents the variation in the PL intensity associated with the formation of 2-hydroxyterepthalic acid as a function of solar-radiation exposure time as obtained for the as-received flyash particles (a) and anatase-TiO₂-coated (cycle-5) SiO₂/γ-Fe₂O₃ magnetic photocatalyst (containing 26 wt. % TiO₂, 25 wt. % γ-Fe₂O₃, and 49 wt. % SiO₂) (b). The excitation wavelength is ˜315 nm.

FIG. 15 represents the variation in q_(e), obtained using the as-received flyash particles, as a function of number of dye-adsorption cycles conducted in the dark-condition. The dye-adsorption cycles 1-4 are conducted before and cycle-5 is conducted after applying the surface-cleaning treatment, conducted under the solar-radiation exposure, to decompose the previously adsorbed-dye from the surface of as-received flyash particles.

FIG. 16 represents the high magnification TEM images (a,b) and EDX spectrum (c) of micro-nano integrated HTN-flyash composite particles. (a,b) show the interphase regions between the flyash particle and surface-adsorbed HTN processed via an ion-exchange mechanism, operating under the dark-condition in an aqueous solution, using the Fe³⁺ cations for the surface-sensitization of as-received flyash particles. The HTN-flyash composite particles (non-magnetic) contain 20 wt. % Fe and 7 wt. % HTN (balance being the weight of flyash particles).

FIG. 17 represents the variation in the normalized concentration of surface-adsorbed MB dye as obtained for the as-received flyash particles (i) and HTN-flyash composite particles (non-magnetic) (ii). The latter is processed by using the Fe³⁺ cations for the surface-sensitization of as-received flyash particles. The initial MB dye concentration and the initial solution-pH are 30 μM and ˜7.5. The HTN-flyash composite particles (non-magnetic) contain 20 wt. % Fe and 7 wt. % HTN (balance being the weight of flyash particles).

FIG. 18 represents the TEM image (a) and EDX spectrum (b) obtained using the HTN-flyash composite particles processed using the H⁺ ions, instead of metal-cations (M^(n+)), for the surface-sensitization of as-received flyash particles. The HTN-flyash composite particles (non-magnetic) containing ˜2 wt. % H and 9 wt. % HTN (balance being the weight of flyash particles).

FIG. 19 represents the variation in the normalized concentration of surface-adsorbed MB dye as a function of contact time as obtained for the as-received flyash particles (i) and HTN-flyash composite particles (non-magnetic) (ii). The latter is processed using the H⁺ ions, instead of metal-cations (M^(n+)), for the surface-sensitization of as-received flyash particles. The initial MB dye concentration and initial solution-pH are 15 μM and ˜7.5. The HTN-flyash composite particles (non-magnetic) containing ˜2 wt. % H and 9 wt. % HTN (balance being the weight of flyash particles).

FIG. 20 represents the TEM image (a), EDX spectrum (b), and XRD pattern (c) obtained using the as-received γ-Fe₂O₃ (maghemite) magnetic particles.

FIG. 21 represents (a-c) the TEM images of nano-nano integrated HTN-γ-Fe₂O₃ magnetic nanocomposite particles, with the corresponding SAED patterns shown as the insets in (a,b), processed via an ion-exchange mechanism operating under the dark-condition in an aqueous solution. In (a) and (b,c), the amount of γ-Fe₂O₃ in the magnetic nanocomposite is 50 wt. % and 10 wt. % respectively (balance being the weight of HTN).

FIG. 22 represents the EDX pattern of HTN-γ-Fe₂O₃ magnetic nanocomposite particles processed via an ion-exchange mechanism operating under the dark-condition in an aqueous solution. The amount of γ-Fe₂O₃ in the magnetic nanocomposite is 10 wt. % (balance being the weight of HTN).

FIG. 23 represents the aqueous solution of MB dye (7.5 μM) (i) and the clear aqueous solution obtained after adsorbing the MB dye using the HTN-γ-Fe₂O₃ magnetic nanocomposite particles, which are settled at the bottom after the dye-adsorption process using a moderate external magnetic field (ii). The amount of γ-Fe₂O₃ in the magnetic nanocomposite is 10 wt. % (balance being the weight of HTN). M′ represents an external magnet and arrow indicates its position.

FIG. 24 represents the variation in the PL intensity associated with the formation of 2-hydroxyterepthalic acid as a function of UV-radiation exposure time as obtained for the magnetic γ-Fe₂O₃ nanoparticles (a), hydrothermally processed HTN (b), and commercially available nanocrystalline anatase-TiO₂ (Central Drug House (CDH) (P) Ltd., New Delhi, India) (c). The excitation wavelength is ˜315 nm.

FIG. 25 represents the variation in the normalized concentration of surface-adsorbed MB dye as a function of contact time as obtained for the HTN-γ-Fe₂O₃ (5 wt. %) magnetic nano-composite particles under the different test-conditions. The initial MB dye concentration varies as 30 μM (i) and 100 μM (ii, iii). The graphs (ii) and (iii) are obtained without and with the surface-cleaning treatment of HTN-γ-Fe₂O₃ (5 wt. %) magnetic nano-composite particles following the 1.2 mg·g⁻¹ of MB dye adsorption on the surface as obtained in the graph (i).

FIG. 26 represents (a) typical TEM image of the Pt-deposited nanocrystalline anatase-TiO₂ photocatalyst having the Pt/Ti mole-ratio of 5% and (b) the variation in the UV-visible absorption intensity obtained after the dye-adsorption experiments conducted using the industry effluent samples containing the Red M5B reactive dye, as observed for the HTN-γ-Fe₂O₃ (10 wt. %) magnetic nano-composite particles under the different test-conditions—(i) initial dye-solution, (ii) 1^(st) cycle of dye-adsorption, (iii) 2^(nd) cycle of dye-adsorption, (iv-vi) 2^(nd) cycle of dye-adsorption following the surface-cleaning treatment conducted using the Pt-deposited nanocrystalline anatase-TiO₂ having the Pt/Ti mole-ratio of 1, 5, and 10% respectively. In (a), the SAED pattern is shown as an inset.

FIG. 27 represents the TEM image (a) and EDX spectrum (b) obtained using the γ-Fe₂O₃-HTN-flyash magnetic nanocomposite. In (a), the inset shows the corresponding SAED pattern. The HTN-γ-Fe₂O₃-flyash magnetic composite particles contains ˜1 wt. % γ-Fe₂O₃ and ˜8 wt. % HTN (balance being the weight of flyash particles).

FIG. 28 represents the digital photographs comparing the ability of a vertically held magnet (M), producing a moderate magnetic field, to hold the as-received flyash particles (a) and HTN-γ-Fe₂O₃-flyash magnetic nanocomposite particles (b). The arrows at the left-hand bottom-corner and right-hand upper-corner indicate the respective position of as-received flyash particles (a) and HTN-γ-Fe₂O₃-flyash magnetic nanocomposite particles (b). The HTN-γ-Fe₂O₃-flyash magnetic composite particles contains ˜1 wt. % γ-Fe₂O₃ and ˜8 wt. % HTN (balance being the weight of flyash particles).

FIG. 29 represents (a) the variation in the normalized concentration of surface-adsorbed MB dye as a function of contact time as obtained for the as-received flyash particles (i) and HTN-γ-Fe₂O₃-flyash magnetic composite particles (ii). The initial MB dye concentration and the initial solution-pH are 7.5 μM and 7.5. (b) represents the aqueous solution of MB dye (7.5 μM) (i) and the clear aqueous solution obtained after adsorbing the MB dye using the HTN-γ-Fe₂O₃-flyash magnetic composite particles, which are settled at the bottom after the dye-adsorption process using a moderate external magnetic field (ii). M′ represents an external magnet and arrow indicates its position. The HTN-γ-Fe₂O₃-flyash magnetic composite particles contain ˜1 wt. % γ-Fe₂O₃ and ˜8 wt. % HTN (balance being the weight of Sn²⁺-sensitized flyash particles);

FIG. 30 represents the TEM image (a) and EDX spectrum (b) obtained using the (10 wt. %) HTN-SnO composite particles.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a method involving both, the surface-sensitization of flyash particles with the metal-cations (M^(n+)) or protons and an ion-exchange mechanism operating under the dark-condition in an aqueous solution, to process an innovative product consisting of the semiconductor-oxides nanotubes-flyash composite particles (magnetic or non-magnetic), and a method to recycle these composite particles in the dye-removal application thereof. The present invention also provides a method, involving an ion-exchange mechanism operating under the dark-condition in an aqueous solution, to process a product consisting of the semiconductor-oxides nanotubes-metal oxide nanocomposite particles (magnetic or non-magnetic), by eliminating the surface-sensitization step, wherein the metal-oxide is essentially the oxide of metal-cation (M^(n+)) which can surface-sensitize the flyash particles; and a method to recycle the magnetic/non-magnetic composite/nanocomposite particles in the dye-removal application thereof. In the present invention, the as-received flyash particles are first surface-sensitized by adsorbing the metal-cations (M^(n+)) on their surfaces, selected from the group consisting of Sn²⁺/Sn⁴⁺, Fe²⁺/Fe³⁺, Pb²⁺, Zn²⁺, Cu²⁺, Mn²⁺, Ti⁴⁺, and others, by stirring the flyash particles in an acidic aqueous solution of metal-salt selected from the group consisting of chloride, nitrate, and sulfate-salts of metal-cations.

The semiconductor-oxides nanotubes (HTN or ATN), which are processed separately via the conventional hydrothermal technique followed by the typical washing-cycles, are added to an acidic aqueous suspension of surface-sensitized flyash particles under the continuous overhead stirring. The nanotubes get deposited on the surface-sensitized flyash particles under the dark-condition in an aqueous solution, forming the nano-micro integrated semiconductor-oxides nanotubes-flyash composite particles (non-magnetic). The formation of composite particles has been attributed to the ability of semiconductor-oxides nanotubes to undergo an ion-exchange mechanism, operating under the dark-condition in an aqueous solution, with the metal-cations (M^(n+)) which are pre-adsorbed on the surface of flyash particles.

The metal-cations (M^(n+)) may be replaced with the protons (H⁺) to attach semiconductor-oxides nanotubes to the surface of sensitized flyash particles an ion-exchange mechanism operating under the dark-condition in an aqueous solution. The nanotubes are, hence, attached or anchored to the surface-sensitized flyash particles typically at the short-edges (tube-openings) due to higher energy of the nanotube-edge relative to that of nanotube-surface. It is obvious that replacing the pure semiconductor-oxides nanotubes with those having the surface-deposited (or surface-decorated) magnetic nanoparticles (metal or metal-oxide) or attached to the magnetic nanoparticles (metal or metal oxide, shown as an oval-shape dark-color particle), the semiconductor-oxides nanotubes-magnetic nanoparticles-flyash composite having the magnetic property can be produced.

The semiconductor-oxides nanotubes-flyash composite particles (magnetic or non-magnetic) (and also the as-received flyash particles) are suitable for the removal of an organic synthetic-dye from an aqueous solution via the surface-adsorption process, involving the ion-exchange and electrostatic-attraction mechanisms operating in the dark-condition, and can be separated from the treated aqueous solution via the centrifuging, gravity-settling, or magnetic separation. It is obvious that the dye-adsorption capacity of semiconductor-oxides nanotubes-flyash composite particles can be increased by using the nanotubes which are doped with the noble-metal(s) or surface-deposited with the noble-metal nanoparticles, selected from the group consisting of Au, Ag, Pt, Pd, and other noble-metals. The previously adsorbed-dye can be decomposed on the surface of semiconductor-oxides nanotubes-flyash composite particles (non-magnetic) (or the as-received flyash particles), via an innovative method, by mixing them with the conventional magnetic photocatalyst, such as the nanocrystalline anatase-TiO₂-coated SiO₂/γ-Fe₂O₃ in an aqueous solution and then subjecting the resulting aqueous suspension to the UV or solar-radiation exposure under the continuous overhead stirring. The photocatalytic activity of semiconductor-oxides nanotubes and as-received flyash particles under the UV or solar-radiation is relatively low. However, relatively large concentration of free-OH. are generated by the nanocrystalline magnetic photocatalyst particles under the similar test-conditions, which attack and degrade an organic synthetic-dye previously adsorbed on the surface of semiconductor-oxides nanotubes-flyash composite particles (non-magnetic) (or the as-received flyash particles). After the decomposition of previously adsorbed-dye on the surface (that is, the surface-cleaning treatment), the semiconductor-oxides nanotubes-flyash composite particles (non-magnetic) (or the as-received flyash particles) and the magnetic photocatalyst can be separated from their mechanical mixture using a moderate external magnetic field. The surface-cleaned semiconductor-oxides nanotubes-flyash composite particles (non-magnetic) (or the as-received flyash particles) can be recycled for the next-cycle of dye-adsorption conducted in the dark-condition. It is obvious that for the recycling of non-magnetic semiconductor-oxides nanotubes-flyash composite particles (or the as-received flyash particles), in the dye-removal application, the nanocrystalline photocatalyst particles must be magnetic for their effective magnetic separation after the dye-decomposition under an exposure to the UV or solar-radiation. It is also obvious that for the recycling of magnetic semiconductor-oxides nanotubes-flyash composite particles, the nanocrystalline photocatalyst particles must be non-magnetic such as the nanocrystalline anatase-TiO₂. Moreover, the magnetic photocatalyst, used for the recycling of semiconductor-oxides nanotubes-flyash composite particles (or the as-received flyash particles), may contain the magnetic component selected from the group of γ-Fe₂O₃, CoFe₂O₄, NiFe₂O₄, MnFe₂O₄, Co, Fe, Ni, and other magnetic materials, and the photocatalyst component selected from the group of nanocrystalline semiconductor materials consisting of TiO₂, ZnO, CdS, ZnS, and others, in the undoped or doped form, without or with the surface-modifications including the deposition of noble-metal or foreign metal-oxide nanoparticles. It is also obvious that if the photocatalyst component of the magnetic photocatalyst is doped with the non-metals such as C, N, S, and others, an exposure to the fluorescent or visible-radiation may also be used to generate the large concentration of free-OH′ for decomposing the previously adsorbed-dye on the surface of semiconductor-oxides nanotubes-flyash composite particles. To produce the semiconductor-oxides nanotubes-metal oxide nanocomposite particles (magnetic or non-magnetic), it is obvious that the flyash particles must be replaced with the metal-oxide nanoparticles. However, if the metal-oxide is the oxide of metal-cation (M^(n+)), which can surface-sensitize the flyash particles, wherein the metal-oxide is selected from the group consisting of γ-Fe₂O₃, SnO/SnO₂, PbO, ZnO, CuO, MnO, TiO₂, and others, then the surface-sensitization is not an essential step and can be eliminated. Typically, to produce a magnetic nanocomposite via an ion-exchange mechanism operating under the dark-condition in an aqueous solution, the as-received flyash particles are replaced with the nanocrystalline magnetic metal-oxide particles such as γ-Fe₂O₃ (maghemite). Since γ-Fe₂O₃ contains Fe²⁺/Fe³⁺ cations which can surface-sensitize the as-received flyash particles, the surface-sensitization step can be eliminated for attaching or anchoring the semiconductor-oxides nanotubes to the surface of magnetic γ-Fe₂O₃ nanoparticles via an ion-exchange mechanism operating under the dark-condition in an aqueous solution.

The semiconductor-oxides nanotubes are attached or anchored to the surface of metal-oxide, such as γ-Fe₂O₃, nanoparticles typically at the short-edges (tube-openings) due to higher energy of the nanotube-edge relative to that of nanotube-surface. The nano-nano integrated semiconductor-oxide nanotubes-metal oxide nanocomposite particles, thus produced, are also suitable for the removal of an organic synthetic-dye from an aqueous solution via the surface-adsorption process, involving the ion-exchange and electrostatic-attraction mechanisms, operating in the dark-condition. Typically, the semiconductor-oxides nanotubes-γ-Fe₂O₃ magnetic nanocomposite particles find application for the removal of an organic synthetic-dye from an aqueous solution via the surface-adsorption process, involving the ion-exchange and electrostatic-attraction mechanisms operating in the dark-condition, and can be separated from the treated aqueous solution using an external magnetic field. The previously adsorbed-dye can be decomposed on the surface of semiconductor-oxides nanotubes-γ-Fe₂O₃ magnetic nanocomposite particles by mixing them with the non-magnetic nanocrystalline particles of anatase-TiO₂ photocatalyst in an aqueous solution and then subjecting the resulting suspension to the UV or solar-radiation under the continuous overhead stirring. It is to be noted that the pure nanocrystalline anatase-TiO₂ used for the surface-cleaning treatment may get attached to (or anchored to) the HTN-γ-Fe₂O₃ magnetic nanocomposite particles via an ion-exchange mechanism operating the under the dark-condition in an aqueous solution. This makes the magnetic separation of photocatalyst particles from the magnetic nanocomposite particles difficult after the surface-cleaning treatment. To avoid this situation, the pure nanocrystalline anatase-TiO₂ is not utilized as a photocatalyst but surface-modified to avoid the operation of ion-exchange mechanism while maintaining (or even increasing) the high concentration of free-OH′ produced under the UV or solar-radiation exposure. The noble-metal-deposited (selected from the group of Pt, Au, Pd, Ag, and others) nanocrystalline anatase-TiO₂ effectively serves both the purposes. The photocatalytic activity of semiconductor-oxides nanotubes and γ-Fe₂O₃ magnetic nanoparticles under the UV or solar-radiation is relatively low. The photocatalytic activity of semiconductor-oxides nanotubes, under the UV or solar-radiation, is further decreased if the previously adsorbed-dye exists on their surfaces. However, relatively large concentration of free-OH′ are generated by the non-magnetic noble-metal deposited nanocrystalline anatase-TiO₂ photocatalyst particles under the UV or solar-radiation, which attack and degrade the organic synthetic-dye previously adsorbed on the surface of semiconductor-oxides nanotubes-γ-Fe₂O₃ magnetic nanocomposite particles. After the decomposition of previously adsorbed-dye on the surface (that is, the surface-cleaning treatment), the semiconductor-oxides nanotubes-γ-Fe₂O₃ magnetic nanocomposite particles and the non-magnetic noble-metal deposited nanocrystalline anatase-TiO₂ photocatalyst particles can be separated from their mechanical mixture using a moderate external magnetic field. The surface-cleaned semiconductor-oxides nanotubes-γ-Fe₂O₃ magnetic nanocomposite particles can be recycled for the next-cycle of dye-adsorption conducted in the dark-condition. It is obvious that for the magnetic nanocomposite particles, the nanocrystalline photocatalyst particles must be non-magnetic, which may be selected from the group consisting of the nanocrystalline (˜3-30 nm) semiconductor materials such as TiO₂, ZnO, CdS, ZnS, and others. It is also obvious that for the recycling of semiconductor-oxides nanotubes-metal oxide nanocomposite particles (non-magnetic), the nanocrystalline photocatalyst particles must be magnetic, that is magnetic photocatalyst, which may contain the magnetic component selected from the group of γ-Fe₂O₃, CoFe₂O₄, NiFe₂O₄, MnFe₂O₄, Co, Fe, Ni and other magnetic materials, and the photocatalyst component selected from the group of nanocrystalline (˜3-30 nm) semiconductor materials consisting of TiO₂, ZnO, CdS, ZnS, and others, in the undoped or doped form, without or with the surface-modifications including the deposition of noble-metal or foreign metal-oxide nanoparticles. It is also obvious that if the photocatalyst particles (magnetic or non-magnetic) are doped with the non-metals such as C, N, S, and others, an exposure to the fluorescent or visible-radiation may be used to generate the large concentration of free-OH. for decomposing the previously adsorbed-dye on the surface of semiconductor-oxides nanotubes-metal Oxide nanocomposite particles. It is obvious that the previously adsorbed-dye on the surface of semiconductor-oxides nanotubes-flyash composite particles (magnetic or non-magnetic) and the semiconductor-oxides nanotubes-metal oxide nanocomposite particles (magnetic or non-magnetic) can be decomposed via the Fenton-like reactions by stirring them in the H₂O₂ solution (3-100 wt. %) typically in the dark-condition. Under this situation, the use of both the photocatalyst particles and an exposure to the UV, solar, fluorescent, or visible-radiation is not essential for the surface-cleaning treatment; that is, the latter can be conducted in the dark-condition. It is also obvious that the different composite products of this invention can also be used to form a static-bed for the dye-removal application, wherein the static-bed is formed using a mechanical mixture of non-magnetic flyash-based composite particles and the magnetic photocatalyst particles or using a mechanical mixture of magnetic nanocomposite particles and the non-magnetic photocatalyst particles. After the dye-adsorption obtained through the passage of aqueous dye-solution thorough the bed-column, the static-bed having the surface-adsorbed dye may be exposed to the UV, solar, fluorescent, or visible-radiation while simultaneously passing an aqueous solution through the bed-column to decompose the previously adsorbed-dye on the surface. The previously adsorbed-dye on the surface of static-bed may also be decomposed, typically in the dark-condition, by passing the H₂O₂ solution (3-100 wt. %) through the bed-column. If the static-bed is formed using only the flyash-based composite products with the absence of photocatalyst particles, then the previously adsorbed-dye on the surface of static-bed may be decomposed, typically in the dark-condition, by passing the H₂O₂ solution (3-100 wt. %) through the bed-column. The present invention provides an innovative method, which involves the surface-sensitization step, for the processing of an innovative product consisting of the nano-micro integrated semiconductor-oxides nanotubes-flyash composite particles (magnetic or non-magnetic), via an ion-exchange mechanism operating under the dark-condition in an aqueous solution, and the industrial dye-removal application involving their recycling via an innovative method thereof; 50-250 ml acidic aqueous solution having the initial solution-pH in the range of ˜1-2, adjusted using the 1 M HCl solution, is first prepared; 5-60 g·l⁻¹ of metal-salt is dissolved in the acidic aqueous solution, wherein the metal-salt is selected from the group consisting of chloride, nitrate, and sulfate-salts of Sn²⁺/Sn⁴⁺, Fe²⁺/Fe³⁺, Pb²⁺, Zn²⁺, Cu²⁺, Mn²⁺, Tr⁺, and other metal-cations (in general, M^(n+)); the dissolution time of metal-salt in the acidic aqueous solution is varied in the range of 30 min-5 h under the continuous overhead stirring; 1-10 g of as-received flyash particles having the spherical morphology and diameter in the range of 0.5-100 μm are then suspended in the acidic aqueous solution of metal-salt; the resulting suspension is stirred continuously using an overhead stirrer for 30 min-5 h for adsorbing the metal-cations (Mn⁺) on the surface of as-received flyash particles (surface-sensitization); the semiconductor-oxides nanotubes such as HTN or ATN are processed separately via the conventional hydrothermal method in combination with either conventional sol-gel or acetic acid (CH₃COOH) modified sol-gel method; 0.05-3 g of HTN or ATN are then added to the above suspension of surface-sensitized flyash particles under the continuous overhead stirring; the suspension is then sonicated for 5-30 min and then stirred continuously using an overhead stirrer for 30 min-5 h for the adsorption of semiconductor-oxides nanotubes on the surface-sensitized flyash particles via an ion-exchange mechanism operating under the dark-condition in an aqueous solution; the nano-micro integrated semiconductor-oxides nanotubes-flyash composite particles (non-magnetic), thus formed, are separated using a centrifuge operated at 2000-4000 rpm, washed multiple-times using 50-250 ml distilled-water till the solution-pH of filtrate remains unchanged or constant, separated using a centrifuge operated at 2000-4000 rpm, and then dried in an oven at 70-90° C. for 10-15 h. The various parameters such as the dissolution time and initial concentration of metal-salt, the contact time of aqueous solution of metal-salt with the as-received flyash particles and that of the semiconductor-oxides nanotubes with the surface-sensitized flyash particles are optimized for the as-received flyash particles surface-sensitized with the Sn²⁺ cations. The as-received flyash particles or the nano-micro integrated semiconductor-oxides nanotubes-flyash composite particles (non-magnetic) are suitable for the removal of an organic synthetic-dye from an aqueous solution via the surface-adsorption process involving the ion-exchange and electrostatic-attraction mechanisms operating in the dark-condition. The dye-adsorption measurements are conducted in the dark-condition using the methylene blue (MB) as a model catalytic dye-agent. The effect of Ag-doping (Ag/Ti ratio is varied as 0, 1, and 5%) on the dye-adsorption capacity of pure-ATN and ATN-flyash composite particles is shown within the initial MB dye concentration range of 5-100 μM at the initial solution-pH within the range of ˜2.5-11. In order to decompose the previously adsorbed-dye on the surface of as-received flyash particles or the semiconductor-oxides nanotubes-flyash composite particles (non-magnetic) and to recycle them as catalyst for the next-cycles of dye-adsorption conducted in the dark-condition, 0.1-5 g of as-received flyash particles or the semiconductor-oxides nanotubes-flyash composite particles (non-magnetic) having the surface-adsorbed MB dye (0.1-10 mg·g⁻¹) are suspended in 50-250 ml aqueous solution under the continuous overhead stirring; 0.1-5 g of conventional magnetic photocatalyst (typically, the nanocrystalline (˜3-30 nm) anatase-TiO₂-coated SiO₂/γ-Fe₂O₃ magnetic particles (processed separately via the combination of modified-Stober and conventional sol-gel methods) are then suspended under the continuous overhead stirring; the resulting suspension is sonicated for 5-30 min and then stirred continuously using an overhead stirrer under the UV or solar-radiation exposure for 1-10 h; the mixture of as-received flyash particles or the semiconductor-oxides nanotubes-flyash composite particles (non-magnetic) and the magnetic photocatalyst particles are separated using a centrifuge operated at 2000-4000 rpm, washed multiple-times using 50-250 ml distilled-water till the solution-pH of the filtrate remained unchanged or neutral; the non-magnetic as-received flyash particles or the semiconductor-oxides nanotubes-flyash composite particles and the magnetic photocatalyst particles are separated using a moderate external magnetic field and then dried in an oven at 70-90° C. for 10-15 h; the dried as-received flyash particles or the semiconductor-oxides nanotubes-flyash composite particles (non-magnetic) are recycled for the next-cycles of, dye-adsorption conducted in the dark-condition; the nano-micro integrated HTN-flyash composite particles are also processed by replacing the Sn²⁺ cations with the Fe³⁺ cations as the surface-sensitizer and iron(III) nitrate (Fe(NO₃)₃.6H₂O) as a metal-salt instead of SnCl₂; the amount of MB dye-adsorbed by the nano-micro integrated HTN-flyash composite particles, processed using the Fe³⁺ cations as the surface-sensitizer, is compared with that adsorbed using the as-received flyash particles at the initial MB concentration within the range of 7.5-100 μM and the initial solution-pH of 2.5-11; the nano-micro integrated HTN-flyash composite particles are also processed by replacing the Sn²⁺ cations with the H⁺ ions as the surface-sensitizer (that is, without the addition of a metal-salt, and hence, eliminating the dissolution time); the amount of MB adsorbed by the nano-micro integrated HTN-flyash composite particles, processed Using the H⁺ ions as the surface-sensitizer, is compared with that adsorbed by the as-received flyash particles at the initial MB concentration within the range of 7.5-100 μM and the initial solution-pH of 2.5-11. The present invention also provides an innovative method without involving the surface-sensitization step for the processing of an innovative product, consisting of the nano-nano integrated semiconductor-oxides nanotubes-metal oxide nanocomposite particles (magnetic or non-magnetic), via an ion-exchange mechanism operating under the dark-condition in an aqueous solution, and the industrial dye-removal application involving their recycling via an innovative method thereof; the metal-oxide nanoparticles are selected from the group of metal-oxides, such as γ-Fe₂O₃ (magnetic), SnO/SnO₂, PbO, ZnO, CuO, MnO, TiO₂, and others, wherein the metal-oxide is essentially the oxide of metal-cation (M^(n+)) which can surface-sensitize the surface of flyash particles; the semiconductor-oxides nanotubes-γ-Fe₂O₃ magnetic nanocomposite particles are processed via an ion-exchange mechanism operating under the dark-condition in an aqueous solution without the involvement of surface-sensitization step since Fe²⁺/Fe³⁺ ions within the γ-Fe₂O₃ magnetic nanoparticles can surface-sensitize the as-received flyash particles; 50-250 ml aqueous suspension is first prepared by suspending 0.1-0.9 g of γ-Fe₂O₃ magnetic nanoparticles are dispersed in an aqueous solution having the neutral solution-pH (˜6.5-7.5) under the continuous overhead stirring; 0.1-0.9 g of HTN is added to this suspension under the continuous overhead stirring; the resulting suspension containing total 1 g of solid particles added is sonicated for 5-30 min and then stirred in the dark-condition for 1-10 h using an overhead stirrer; the HTN-γ-Fe₂O₃ magnetic nanocomposite particles, thus formed via an ion-exchange mechanism operating under the dark-condition in an aqueous solution, are separated using the moderate external magnetic field and washed multiple-times using the distilled-water till the solution-pH of filtrate remains unchanged or neutral; the magnetic nanocomposite particles are separated again using the moderate external magnetic field and dried in an oven at 70-90° C. for 10-15 h. 0.1-5.0 g of semiconductor oxide nanotubes-γ-Fe₂O₃ magnetic nanocomposite particles, containing 5-90 wt. % of HTN, are then used for the removal of MB dye (7.5-250 μM) from an aqueous solution (50-250 ml) via the surface-adsorption process, involving the ion-exchange and electrostatic-attraction mechanisms operating in the dark-condition.

To decompose the previously adsorbed-dye on the surface and to recycle them for the next-cycles of dye-adsorption conducted in the dark-condition, 0.1-5.0 g of magnetic HTN-γ-Fe₂O₃ nanocomposite particles, containing 1-5 mg·g⁻¹ of MB dye adsorbed on the surface, are suspended in 50-250 ml aqueous solution under the continuous overhead stirring; the non-magnetic Pt-deposited (Pt/Ti mole ratio within the range of 1-15%) nanocrystalline (˜10-30 nm) anatase-TiO₂ photocatalyst particles are then added to the above suspension under the continuous overhead stirring; the resulting suspension is sonicated for 5-30 min, then exposed to the UV or solar-radiation for 30 min-5 h under the continuous overhead stirring; the mixture of surface-cleaned magnetic nanocomposite particles and the non-magnetic Pt-deposited nanocrystalline anatase-TiO₂ photocatalyst particles are separated using a centrifuge operated at 2000-4000 rpm and washed multiple-times using 50-250 ml distilled-water till the solution-pH of filtrate remains unchanged; the surface-cleaned magnetic nanocomposite particles are separated from the non-magnetic Pt-deposited nanocrystalline anatase-TiO₂ photocatalyst using an external magnetic field; the separated powders are dried in an oven at 70-90° C. for 10-15 h; the surface-cleaned magnetic nanocomposite particles are recycled for the next-cycle of dye-adsorption conducted in the dark-condition and its dye-adsorption behavior is compared with that of magnetic nanocomposite particles having the surface-adsorbed MB dye which is not subjected to the surface-cleaning treatment.

The present invention also provides an innovative method for the processing of an innovative product consisting of flyash-based magnetic composite particles; the HTN-γ-Fe₂O₃ magnetic nanocomposite (5-90% HTN) is first processed, without involving the surface-sensitization step, via the ion-exchange mechanism operating under the dark-condition in an aqueous solution; the HTN-γ-Fe₂O₃ magnetic nanocomposite is then attached or anchored to the flyash particles, surface-sensitized with Sn²⁺ cations, via the ion-exchange mechanism operating under the dark-condition in an aqueous solution.

The non-obvious inventive step(s) of the present invention with respect to the prior art are as follows.

-   (1) The addition of hydrothermally processed semiconductor-oxide     nanotubes, such as the HTN or ATN, in the aqueous suspension of     flyash particles having the pre-adsorbed surface metal-cations. -   (2) The formation of micro-nano integrated semiconductor-oxide     nanotubes-flyash composite particles via an ion-exchange mechanism     operating under the dark-condition in an aqueous solution. -   (3) The use of micro-nano integrated semiconductor-oxide     nanotubes-flyash composite particles (or the as-received flyash     particles) in the dye-removal application via the surface-adsorption     process involving the ion-exchange and electrostatic-attraction     mechanisms operating in the dark-condition, wherein the composite     particles (or the as-received flyash particles) are recycled by     decomposing the previously adsorbed-dye on the surface via an     innovative process involving the use of combination of a magnetic     photocatalyst and an exposure to the UV or solar-radiation. -   (4) The addition of hydrothermally processed semiconductor-oxide     nanotubes, such as the HTN or ATN, in an aqueous suspension of     metal-oxide particles at the neutral solution-pH, wherein the     metal-oxide nanoparticles contain the metal-cations (Mn+) which can     surface-sensitize the flyash particles. This results in the     formation of semiconductor-oxide nanotubes-metal oxide     nano-composite (magnetic or non-magnetic) particles without     involving the surface-sensitization step. -   (5) The formation of nano-nano integrated semiconductor-oxide     nanotubes-metal oxide nanocomposite particles (magnetic or     non-magnetic) via an ion-exchange mechanism operating under the     dark-condition in an aqueous solution without involving the     surface-sensitization step. -   (6) The use of nano-nano integrated semiconductor-oxide     nanotubes-metal oxide nanocomposite particles (magnetic) in the     dye-removal application via the surface-adsorption process involving     the ion-exchange and electrostatic attraction mechanisms operating     in the dark-condition, wherein the magnetic nanocomposite particles     are recycled by decomposing the previously adsorbed-dye on the     surface via an innovative process involving the use of combination     of non-magnetic noble-metal-deposited (including Pt, Au, Pd, Ag, and     others) photocatalyst, such as nanocrystalline anatase-TiO₂, and     exposure to the UV or solar-radiation. -   (7) The adsorption of semiconductor-oxide nanotubes, which are     attached to (or anchored to) the magnetic metal-oxide nanoparticles,     on the surface-sensitized flyash particles via an ion-exchange     mechanism operating under the dark-condition in an aqueous solution     resulting in the formation of flyash-based magnetic composite     particles.

The novelty of the present invention with respect to the prior art is as follows.

-   (1) The formation of micro-nano integrated semiconductor-oxide     nanotubes-flyash composite particles via an ion-exchange mechanism     operating under the dark-condition in an aqueous solution. -   (2) The formation of nano-nano integrated semiconductor-oxide     nanotubes-metal oxide nanocomposite particles via an ion-exchange     mechanism operating under the dark-condition in an aqueous solution     by eliminating the surface-sensitization step. -   (3) The recycling of semiconductor-oxide nanotubes-flyash composite     particles (non-magnetic) in the dye-removal application involving     the use of a magnetically separable and photocatalytically active     magnetic photocatalyst. -   (4) The recycling of semiconductor-oxide nanotubes-metal oxide     nanocomposite particles (magnetic) in the dye-removal application     involving the use of non-magnetic and photocatalytically active     noble-metal-deposited nanocrystalline anatase-TiO₂ particles. -   (5) The formation of micro-nano integrated semiconductor-oxide     nanotubes-metal oxide-flyash composite particles (magnetic) via an     ion-exchange mechanism operating under the dark-condition in an     aqueous solution. -   (6) The use of micro-nano integrated semiconductor-oxide     nanotubes-metal oxide-flyash composite particles (magnetic) in the     application related to the dye-removal via the surface-adsorption     process involving the ion-exchange and electrostatic attraction     mechanisms; and their recycling involving the use of non-magnetic     and photocatalytically active noble-metal-deposited nanocrystalline     anatase-TiO₂ particles.

EXAMPLES

The following examples are given by way of illustration therefore should not be construed to limit the scope of the invention.

Example-1

In this example, HTN are processed via the conventional hydrothermal method. 3 g of as-received nanocrystalline anatase-TiO₂ (Central Drug House (CDH) (P) Ltd., New Delhi, India) is suspended in a highly alkaline aqueous solution, containing 10 M NaOH (Assay 97%, S.D. Fine-Chem Ltd., Mumbai, India), filled up to 84 vol. % of a Teflon-beaker placed in a stainless-steel (SS 316) vessel of 200 ml capacity. The process is carried out with continuous stirring in an autoclave (Amar Equipment Pvt. Ltd., Mumbai, India) at 120° C. for 30 h under an autogenous pressure. The autoclave is allowed to cool naturally to room temperature and the hydrothermal product is separated by decanting the top solution. The initial product is subjected to typical washing-cycles with the first-cycle of washing conducted using 100 ml of 1 M HCl solution (35 wt. %, Qualigens Fine Chemicals, India) at 25° C. for 1 h followed by that using 100 ml of distilled-water for 1 h. The product obtained is then subjected to the second washing-cycle consisting of washing using 100 ml of 1 M HCl at 25° C. for 1 h and then multiple times (#8) using 100 ml of distilled-water at 25° C. for 1 h till the pH (Hanna HI 2210 Bench Top, Sigma-Aldrich, India) of the filtrate became almost constant or neutral. The washed-product is then separated from the solution using a centrifuge (Hettich EBA 20, Sigma-Aldrich, India) and dried in an oven at 80° C. for 12 h to obtain the hydrothermally processed HTN.

The TEM images of as-received flyash particles (National Thermal Power Corporation (NTPC), Ramagundam, India) and the hydrothermally processed HTN are presented in FIGS. 1( a) and 1(b). The as-received flyash particles are spherical with the size in the range of 0.5-2 μM. The average length, internal and outer diameters, and wall-thickness of HTN are 75, 2.3, 4.2, and 0.96 nm respectively. The powder XRD patterns of as-received flyash and HTN are presented in FIGS. 2( a) and 2(b). In FIG. 2( a), the major-peaks of SiO₂, Al₂O₃, and Fe₂O₃ have been identified after comparison with the JCPDS card nos. 83-2465, 78-2426, and 84-0311 respectively. In FIG. 2( b), the XRD pattern has been identified to be similar to that of H₂Ti₃O₇ or the lepidocrocite-type hydrogen titanate (H_(x)Ti_(2-x/4yx/4)O₄.H₂O, where x≈0.7, γ: vacancy). The EDX analyses of as-received flyash and HTN are presented in FIGS. 3( a) and 3(b). In FIG. 3( a), Si, Al, Ca, Fe are observed with the trace amount of Ti (Cu is originating from the Cu-grid used for the TEM analysis). This suggests that the as-received flyash particles are composed of mixture of SiO₂, Al₂O₃, Fe₂O₃, CaO, with the trace amount of TiO₂. On the other hand, in FIG. 3( b), only Ti and O are seen as the major elements within the HTN. By merely stirring the aqueous suspension of mechanically mixed hydrothermally processed HTN and the as-received flyash particles, the former could not be adsorbed on (or attached to or anchored to) the surface of latter. In order to deposit HTN on the surface of as-received flyash particles, the following procedure has been adopted. 40 g·l⁻¹ of tin(II) chloride (SnCl₂, 97%, S.D. Fine-Chem, Mumbai, India) is dissolved in 100 ml of distilled-water having the initial solution-pH ˜1.5 which is adjusted using the 1 M HCl solution under the continuous overhead stirring (Eurostar Digital, IKA, Germany) at 25° C. The dissolution time of SnCl₂ is 2 h. 1.0 g of as-received flyash particles is added to this clear-solution and the resulting suspension is stirred continuously at 25° C. for 4 h using an overhead stirrer to obtain the Sn²⁺-sensitized flyash particles. 0.1 g of hydrothermally processed HTN are then added and the resulting suspension is sonicated (Bandelin Ultrasonic Bath, Aldrich-Labware, Bangalore, India) for 10 min followed by continuous stirring using an overhead stirrer at 25° C. for 4 h. The HTN are adsorbed on the surface of Sn²⁺-sensitized flyash particles via an ion-exchange mechanism operating under the dark-condition in an aqueous solution. The resulting micro-nano integrated HTN-flyash composite particles (non-magnetic) are separated from the aqueous solution using a centrifuge (Hettich EBA 20, Sigma-Aldrich, India) operated at 3000 rpm. The separated HTN-flyash composite particles are then washed using 100 ml of distilled-water for 1 h multiple-times till the pH of filtrate remains unchanged or neutral. The washed composite particles are then dried in an oven at 80° C. for 12 h to obtain the HTN-flyash composite particles (non-magnetic) containing 69 wt. % Sn and 3 wt. % HTN (balance being the weight of flyash particles). The high magnification TEM images of the surface of as-received flyash particle and the interphase boundary within the micro-nano integrated HTN-flyash composite particle (non-magnetic) are shown in FIGS. 4( a) and 4(b). The surface of as-received flyash particle at very high magnification appears to be plain and featureless, FIG. 4( a). In FIG. 4( b), the interface boundary of HTN-flyash composite particle (non-magnetic) is clearly visible. The HTN are seen to be attached to or anchored to the surface of Sn²⁺-sensitized flyash particles typically at the short-edges (or tube-openings) resulting in the formation of micro-nano integrated HTN-flyash composite particles (non-magnetic). The mechanism of HTN adsorption on the surface of Sn²⁺-sensitized flyash particles is an ion-exchange process operating under the dark-condition in an aqueous solution. Since the as-received flyash particles are stirred in an acidic SnCl₂ solution, large amount of Sn²⁺ ions are first adsorbed on the surface of as-received flyash particles. When the hydrothermally processed HTN are added to an aqueous suspension containing the Sn²⁺-sensitized flyash particles, the former tend to pick-up the Sn²⁺ ions, which are pre-adsorbed on the surface of flyash particles, for an exchange with the H⁺ ions in their structure. However, since the Sn²⁺ ions are already chemisorbed on the surface of as-received flyash particles and cannot leave the surface, the HTN get adsorbed on the surface of Sn²⁺-sensitized flyash particles. Since the nanotubes-edge has higher energy than that of nanotubes-surface, the anchoring of HTN on the surface of Sn²⁺-sensitized flyash particles primarily occurs at the shorter-edges (or the nanotubes-openings), FIG. 4( b), instead of along the length of nanotubes. The XRD pattern, FIG. 5( a), and the EDX spectrum, FIG. 5( b), further confirm the formation of HTN-flyash composite particles via an ion-exchange mechanism operating under the dark-condition in an aqueous solution.

Example-2

In this example, the values of different parameters used for the processing of micro-nano integrated HTN-flyash composite particles are identical with those already described in the Example 1 except for the following changes. The initial concentration of SnCl₂ and its dissolution time are varied as 5, 20, 40, and 60 g·l⁻¹ and 30 min, 2 and 4 h respectively. With these processing parameters, the HTN-flyash composite particles (non-magnetic) containing 44, 53, 69, and 77 wt. % Sn and 2, 3, 4, and 7 wt % HTN respectively (balance being the weight of flyash particles) are obtained for the complete dissolution of SnCl₂ (dissolution time of 2 and 4 h).

The HTN-flyash composite particles processed under these conditions are utilized in the dye-adsorption experiments which are conducted at the neutral initial solution-pH of ˜7.5 and in the dark-condition using the MB (methylene blue) as a model catalytic dye-agent. 125 ml aqueous solution is prepared by dissolving 15 μM of MB dye. 4.0 g·l⁻¹ of HTN-flyash composite particles are then dispersed in the MB dye solution and the resulting suspension is stirred continuously in the dark-condition for 180 min using an overhead stirrer. 8 ml aliquot is separated after each 10 or 30 min time interval for obtaining an absorption spectrum, using the UV-visible absorption spectrophotometer (UV-2401 PC, Shimadzu, Japan), of the filtrate obtained after separating the HTN-flyash composite particles using a centrifuge. The normalized concentration of surface-adsorbed MB dye is calculated using the equation of form,

$\begin{matrix} {{{\% \mspace{14mu} {MB}_{adsorbed}} = {\left( \frac{C_{0} - C_{t}}{C_{0}} \right)_{MB} \times 100}},} & (1) \end{matrix}$

which is equivalent of the form,

$\begin{matrix} {{\% \mspace{14mu} {MB}_{adsorbed}} = {\left( \frac{A_{0} - A_{t}}{A_{0}} \right)_{MB} \times 100}} & (2) \end{matrix}$

where, C₀ (mg·l⁻¹) and C_(t) (mg·l⁻¹) correspond to the initial MB dye concentration at the start and after the contact time t with the corresponding absorbance of A₀ and A_(t).

The variation in the normalized concentration of surface-adsorbed MB dye as a function of contact time, as obtained for the micro-nano integrated HTN-flyash composite particles, is presented in FIG. 6( a)-6(c) for the different values of initial SnCl₂ concentration and its dissolution time. It is noted that, in general, the micro-nano integrated HTN-flyash composite particles show higher MB dye-adsorption on the surface relative to that of as-received flyash particles. The Sn²⁺-cations remaining in the aqueous solution, typically at lower dissolution time of SnCl₂ and its higher concentrations, are seen to affect the adsorption of HTN on the surface of Sn²⁺-sensitized flyash particles. The amount of MB dye adsorbed on the surface of micro-nano integrated HTN-flyash composite particles per unit mass of adsorbent (q_(e)) is calculated using the relationship of form,

$\begin{matrix} {q_{e} = \frac{\left( {C_{0} - C_{e}} \right) \times V}{m_{FA}}} & (3) \end{matrix}$

where, C₀ (mg·l⁻¹) is the MB dye concentration within the solution at the equilibrium (that is, after the contact time of 180 min), V (I) the initial volume of MB dye solution, and m_(HTN-FA) (g) the mass of micro-nano integrated HTN-FA composite particles used as dye-adsorbent. The obtained variation in q_(e) as a function of initial SnCl₂ concentration, obtained for the different dissolution time of SnCl₂, is presented in FIG. 7. The as-received flyash particles exhibit the q_(e) of 0.3 mg·g⁻¹. On the other hand, the maximum q_(e) (q_(m)) of 1.2 mg·g⁻¹ is observed for the initial SnCl₂ concentration of 20 g·l⁻¹ and the dissolution time of 2 h, which suggests 4 times increase in the dye-adsorption capacity, at the solution-pH of ˜7.5, due to the deposition of hydrothermally processed HTN on the surface of as-received flyash particles via an ion-exchange mechanism operating in the dark-condition.

Thus, under the given test-conditions, the HTN-flyash composite particles exhibit higher MB dye adsorption capacity than that of as-received flyash particles. The initial SnCl₂ concentration of 20 g·l⁻¹ and the dissolution time of 2 h are determined to be the most optimum conditions leading to the maximum MB dye-adsorption on the surface of HTN-flyash composite particles in the dark-condition. Since in the literature, the capacity of as-received flyash particles for the adsorption of cationic MB dye is shown to be insensitive to the initial solution-pH; while, that of hydrothermally processed HTN is shown to be drastically enhanced and reached its maximum value at the initial solution-pH of ˜10 (within the range of ˜2.5-11), it is obvious that the difference in the dye-adsorption capacity of HTN-flyash composite particles (non-magnetic) and that of as-received flyash particles, at the initial solution-pH of ˜10, would be the highest and more than that observed at the initial solution-pH of ˜7.5.

Example-3

In this example, the values of different parameters used for the processing of micro-nano integrated HTN-flyash composite particles are identical with those already described in the Example 1 except for the following changes. The initial SnCl₂ concentration is changed to 20 g·l⁻¹. Both the contact time of SnCl₂ solution with the suspended as-received flyash particles and that of the hydrothermally processed HTN with the Sn²⁺-sensitized flyash particles are varied in the range of 1-4 h at the interval of 1 h. The HTN-flyash composite particles processed under these conditions contain 53 wt. % Sn and 4 wt. % HTN (balance being the weight of flyash particles) and are utilized in the MB dye-adsorption experiments, conducted in the dark-condition, as described above in the Example-2.

The obtained variation in the normalized concentration of surface-adsorbed MB dye as a function of contact time, as obtained for the micro-nano integrated HTN-flyash composite particles, for the different contact time of SnCl₂ solution with the as-received flyash particles, FIG. 8( a), and that of the HTN with the Sn²⁺-sensitized flyash particles, FIG. 8( b). It is noted that, for both the contact time within the range of 1-4 h, the MB dye adsorption remains almost 100%. Hence, within the investigated range of parameters, the contact time of SnCl₂ solution with the as-received flyash particles and that of the HTN with the Sn²⁺-sensitized flyash particles do not have any significant effect on the amount of MB dye adsorbed on the surface of HTN-flyash composite particles.

Thus, considering that higher amount of HTN (greater than ˜10%) could be adsorbed on the surface of Sn²⁺-sensitized flyash particles, the optimum contact time of SnCl₂ solution with the as-received flyash particles and that of HTN with the Sn²⁺-sensitized flyash particles are determined to be 4 h.

Example-4

In this example, the values of different parameters used for the processing of micro-nano integrated HTN-flyash composite particles are identical with those already described in the Example 1 except that the initial concentration of SnCl₂ is 20 g·l⁻¹. The HTN-flyash composite particles processed under these conditions contain 53 wt. % Sn and 4 wt. % HTN (balance being the weight of flyash particles) are utilized in the MB dye-adsorption experiments, conducted in the dark-condition, as described earlier in the Example-2 except that the initial MB dye concentration is varied in the range of 15-90 μM.

The variation in the normalized concentration of surface-adsorbed MB dye as a function of contact time, as obtained for the micro-nano integrated HTN-flyash composite particles, for the different initial MB dye concentration, is presented in FIG. 9( a). The amount of surface-adsorbed MB dye, at equilibrium, is seen to decrease with increasing initial MB dye concentration. The corresponding variation in q_(e) as a function of initial MB dye concentration is presented in FIG. 9( b). The maximum value of q_(e)(q_(m)), under these test-conditions, is noted be 4 mg·g⁻¹, which is 13 times higher than that (0.3 mg·g⁻¹) of as-received flyash particles as determined under the similar test-conditions.

Thus, under the neutral initial solution-pH of ˜7.5, the dye-adsorption capacity of HTN-flyash composite particles is higher than that of as-received flyash particles. It is obvious that by changing the initial solution-pH within the range of 7.5-11, the dye-adsorption capacity of HTN-flyash composite would be further enhanced (typically at the initial solution-pH of ˜10) relative to that of as-received flyash.

Example-5

In this example, Ag-doped ATN with varying Ag/Ti mole-ratio (0, 1, and 5%) are synthesized via the conventional hydrothermal method as already described in the Example 1 except that the as-received anatase-TiO₂ precursor is replaced with the Ag-doped anatase-TiO₂ processed with varying Ag/Ti mole-ratio (0, 1, and 5%). The latter is processed via the conventional sol-gel method modified using the acetic acid (CH₃COOH) as a catalyst. The molar-ratio of CH₃COOH catalyst to Ti(OC₃H₅)₄ precursor, involved in the modified sol-gel process, is 10. The molar-ratio of water to alkoxide-precursor (R-value) is 90. The CH₃COOH-catalyst is essential in the modified sol-gel process to dissolve Ag into the TiO₂ lattice without causing the anatase-to-rutile phase transformation during the calcination treatment which is conducted at higher temperature at 600° C. for 2 h. In contrast to the formation of HTN as described in the Example 1, the nanotubes formed in this example are observed to be ATN, FIG. 10. The pure and Ag-doped ATN processed under these conditions are then utilized in the MB dye-adsorption experiments, conducted in the dark-condition, as described earlier in the Example-2 except that the initial MB dye concentration is varied in between 7.5-250 μM.

The obtained variation in q_(e) as a function of initial MB dye concentration, as obtained using the pure and Ag-doped ATN with varying Ag/Ti mole-ratio (0, 1, and 5%), is presented in FIG. 11( a). The maximum value of q_(e) (q_(m)) is noted be 38 mg·g⁻¹ as observed for the Ag-doped ATN having 1% Ag/Ti mole-ratio. Hence, it is obvious that Ag-doped ATN-flyash composite particles would exhibit higher MB dye-adsorption capacity than that of pure ATN-flyash composite particles, which is demonstrated in FIG. 11( b). Hence, the doping of noble-metal increases the dye-adsorption capacity of ATN (or HTN) and also that of semiconductor-oxide nanotubes-flyash composite particles.

It is obvious that doping ATN or HTN with other noble-metals such as Au, Pt, and Pd as well as non-noble metals such as Gd, Zn, Mn; Cu, and others, would also increase the dye-adsorption capacity of ATN (or HTN)-flyash composite particles. It is also obvious that the other techniques including the surface-deposition of noble-metal catalyst nanoparticles, such as Au, Pt, Pd, Ag, and others, on the surface of ATN (or HTN)-flyash composite particles would increase their dye-adsorption capacity.

Example-6

In this example, the anatase-TiO₂-coated (cycle-5) SiO₂/γ-Fe₂O₃ magnetic nanocomposite particles are first prepared via the modified-Stober and the conventional sol-gel processes. To 2 g suspension of nanocrystalline magnetic γ-Fe₂O₃ particles dispersed in 250 ml of ethanol, 14.5 ml of tetraethylorthosilicate (TEOS, 98%, Sigma-Aldrich, India) is slowly added and stirred for 1 h using an overhead stirrer. This is followed by the drop-wise addition of mixture of 2.3 ml of NH₄OH (25 wt. %, Qualigens Fine Chemicals, India) and 63.4 ml of distilled-water and the suspension is stirred for 12 h. The resulting product is collected via magnetic separation (magnetic separator, Sigma-Aldrich Labware, Bangalore, India), washed first with 100 ml of ethanol and four times with distilled-water followed by drying in an oven at 80° C. for 12 h. By this process, the SiO₂/γ-Fe₂O₃ magnetic nanocomposite particles containing 66 wt. % SiO₂ and 34 wt. % γ-Fe₂O₃ are obtained.

In order to deposit the nanocrystalline anatase-TiO₂, 2 g of SiO₂/γ-Fe₂O₃ magnetic nanoparticles are suspended in a solution of 0.5 g of titanium(IV) iso-propoxide (Ti(OC₃H₇)₄, 97%, Sigma-Aldrich, Bangalore, India) dissolved in 125 ml 2-propanol. To this suspension, a solution consisting 0.15 ml of distilled water (R=5, defined as the ratio of molar concentration (final) of water to that of alkoxide-precursor) dissolved in 125 ml of 2-propanol, was added drop wise. The resulting suspension is stirred for 12 h and the magnetically separated powder is then washed with 100 ml of 2-propanol and then dried in an oven at 80° C. overnight. The sol-gel process is repeated for total 5 cycles to control the thickness of amorphous-TiO₂ coating which is then converted to anatase-TiO₂ via the calcination treatment (heating rate=3° C.·min⁻¹) of the dried-powder conducted at 600° C. for 2 h. The anatase-TiO₂-coated (cycle-5) SiO₂/γ-Fe₂O₃ magnetic nanocomposite, thus obtained, contains 26 wt. % TiO₂, 25 wt. % γ-Fe₂O₃, and 49 wt. % SiO₂ and is referred here as a “magnetic photocatalyst”. The corresponding SEM image, EDX spectrum, and XRD pattern of magnetic photocatalyst are presented in FIG. 12( a)-12(c) respectively. Being magnetic in nature, the anatase-TiO₂-coated (cycle-5) SiO₂/γ-Fe₂O₃ magnetic photocatalyst can be separated from an aqueous solution using an external magnetic field as demonstrated in FIG. 13. (Note: γ-Fe₂O₃ is a ferrimagnetic material with the saturation magnetization of 74 emu·g⁻¹).

The free-OH. trapping experiments are performed, using the terepthalic acid (TA, 98%, Sigma-Aldrich Chemicals, Bangalore, India), which are produced under the continuous solar-radiation exposure of two separate aqueous suspensions containing the suspended particles of as-received flyash particles or the HTN-flyash composite particles (non-magnetic) and the magnetic photocatalyst. 500 μM of TA and 2000 μM of NaOH (assay 97%, S.D. Fine-Chem, Mumbai, India) are first dissolved in 125 ml of aqueous solution. Either 3.2 g·l⁻¹ of magnetic photocatalyst or 24 g·l⁻¹ of as-received flyash particles (or HTN-flyash composite particles) are suspended in this solution. The resulting suspension is stirred continuously using an overhead stirrer under the solar-radiation exposure for 5 h. The free-OH. produced by the as-received flyash particles (or the HTN-flyash composite particles) and the magnetic photocatalyst particles, under the solar-radiation exposure, are trapped by the TA resulting in the formation of 2-hydroxyterephthalic acid. The aliquots are collected at the 1 h time interval and the solid-particles are separated using either a centrifuge or a magnetic separator. The filtrate is analyzed to obtain the photoluminescence (PL) spectra of 2-hydroxyterephthalic acid which exhibits a characteristic PL peak located at ˜625 nm, the intensity of which is recorded as a function of UV-radiation exposure time using the spectrofluorometer (Cary Eclipse, Varian, The Netherlands) at an excitation wavelength of ˜315 nm. The intensity of PL peak is regarded as the measure of amount of free-OH. produced by the catalyst-particles at a given time under the solar-radiation exposure.

The variation in the PL intensity of 2-hydroxyterepthalic acid as a function of solar-radiation exposure Time, as observed for the as-received flyash particles (or HTN-flyash composite particles) and the magnetic photocatalyst, is presented in FIGS. 14( a) and 14(b). It is observed that, for both the samples, the concentration of free-OH. produced increases with increasing solar-radiation exposure time. However, the comparison shows that the as-received flyash particles (or HTN-flyash composite particle) do not generate appreciable amount of free-OH. under the solar-radiation exposure. On the other hand, large amount of free-OH. are produced by the magnetic photocatalyst. This suggests that the magnetic photocatalyst should possess better photocatalytic activity under the solar-radiation exposure than that of the as-received flyash particles (or HTN-flyash composite particles). Since the latter has relatively higher dye-adsorption capacity under the dark-condition, it appears that the previously adsorbed-dye can be easily decomposed on the surface on as-received flyash particles (or HTN-flyash composite particles) by the attack of free-OH′ produced by the magnetic photocatalyst provided both the materials are mechanically mixed and suspended in the same aqueous dye-solution which is exposed to the solar-radiation. It further appears that from the mechanical mixture of as-received flyash particles (or HTN-flyash composite particles) and the magnetic photocatalyst, both the powders can be separated from their mechanical mixture using an external magnetic field. This allows the recycling of as-received flyash particles (or HTN-flyash composite particles) for the next-cycle of dye-adsorption conducted in the dark-condition using the magnetic photocatalyst for their surface-cleaning treatment.

In order to demonstrate this, the as-received flyash particles are used for the multiple MB dye-adsorption cycles (#4) conducted in the dark-condition. All experimental parameters used for these dye-adsorption measurements are identical to those described earlier the Example 2 except for the following changes. The initial concentration of MB dye and that of adsorbent are 7.5 μM and 24 g·l⁻¹. In order to remove the previously adsorbed MB dye from the surface, 3 g of as-received flyash particles with the surface-adsorbed MB dye (0.24 mg·g⁻¹), as obtained after the cycle-4 of dye-adsorption as mentioned above, is first suspended in 125 ml of distilled-water under continuous overhead stirring. Then, 1 g of anatase-TiO₂-coated (cycle-5) SiO₂/γ-Fe₂O₃ magnetic photocatalyst (containing 26 wt. % TiO₂, 25 wt. % γ-Fe₂O₃, and 49 wt. % SiO₂) is added to the above suspension under the continuous overhead stirring. The resulting suspension is stirred continuously under the solar-radiation exposure for 6 h, which results in the decomposition of MB dye adsorbed on the surface of as-received flyash particles due to the large concentration of free-OH. radicals produced by the magnetic photocatalyst under these test-conditions. This is then followed by the separation of surface-cleaned non-magnetic flyash particles mixed with the magnetic photocatalyst particles using a centrifuge operated at 3000 rpm and the washing of mixed solid particles using 100 ml of distilled-water for 1 h multiple-times till the pH of filtrate remains unchanged or neutral. The magnetic and non-magnetic components are then separated using an external magnetic field and dried in an oven at 80° C. for 12 h. The surface-cleaned flyash particles, hence, could be recycled for the next-cycle of dye-adsorption conducted in the dark-condition as demonstrated in FIG. 15. The q_(e) is noted decrease with increasing number of dye-adsorption cycles conducted in the dark-condition. The surface-cleaning treatment, as described above, is applied after the dye-adsorption cycle-4, to decompose the previously adsorbed-dye from the surface, which results in a drastic increase in the dye-adsorption capacity of as-received flyash particles which is comparable with that observed in the cycle-1.

Thus, the recycling of as-received flyash particles is successfully achieved here by decomposing the previously-adsorbed dye from their surfaces by mixing them with a magnetic photocatalyst (typically anatase-TiO₂-coated (cycle-5) SiO₂/γ-Fe₂O₃) in an aqueous solution and exposing the resulting suspension to the solar-radiation. It is obvious that the other “core-shell” type magnetic photocatalysts having different magnetic core such as CoFe₂O₄, NiFe₂O₄, MnFe₂O₄, Ni, Fe, Co, and others, in combination with the shell of other nanocrystalline semiconductor photocatalyst such as ZnO, CdS, ZnS, and others, are also suitable for recycling the as-received flyash particles and the micro-nano integrated HTN-flyash composite particles for the dye-removal application. The magnetic photocatalyst having the morphology other than the “core-shell” structure, such as the nanoparticles of magnetic material deposited on the surface of semiconductor-oxide nanotubes may be also used.

Example-7

In this example, the values of different parameters used for the processing of micro-nano integrated HTN-flyash composite particles are identical with those already described in the Example 1 except for the following changes. The SnCl₂ precursor having the initial concentration of 40 g·l⁻¹ is replaced with Fe(NO₃)₃.9H₂O precursor having the initial concentration of 20 g·l⁻¹, for surface-sensitizing the as-received flyash particles with Fe³⁺ cations instead of Sn²⁺. By this process, the HTN-flyash composite particles (non-magnetic) containing 20 wt. % Fe and 7 wt. % HTN (balance being the weight of flyash particles) are obtained.

The TEM images and the EDX pattern as obtained using the HTN-flyash composite particles, with the surface-sensitization of flyash particles obtained using the Fe³⁺ cations, are presented in FIGS. 16( a)-(b) and 16(c). Similar to the observation made in FIG. 4( b), the HTN are noted to be attached to or anchored to the surface of flyash particles, surface-sensitized with the Fe³⁺ cations, typically at the short-edges (tube-openings) which confirms the operation of ion-exchange mechanism operating under the dark-condition in an aqueous solution. The comparison of EDX spectrum with that of as-received flyash particles, FIG. 3( a), show an increase in the intensity of Fe and Ti peaks, suggesting the formation of HTN-flyash composite particles via the Fe³⁺ sensitization.

The as-received flyash particles and the HTN-flyash composite particles are then utilized in the MB dye-adsorption experiments, conducted in the dark-condition, as described earlier in the Example-2 except that the initial MB dye concentration of 30 μM is used in this example. The variation in the normalized concentration of surface-adsorbed MB dye as a function of contact time, as obtained for the as-received flyash particles and micro-nano integrated HTN-flyash composite particles (latter surface-sensitized with Fe³⁺ ions), is presented and compared in FIG. 17. Relative to the equilibrium MB dye adsorption exhibited by the as-received flyash particles, the amount of MB dye adsorbed at equilibrium is seen to enhance due to the anchoring of HTN on the surface of flyash particles which are pre-sensitized with the Fe³⁺ cations.

Example-8

In this example, the values of different parameters used for the processing of micro-nano integrated HTN-flyash composite particles are identical with those already described in the Example 1 except for the following changes. The SnCl₂ precursor is not utilized for surface-sensitizing the as-received flyash particles with Sn²⁺ cations. The surface-sensitization of as-received flyash particles is achieved using the adsorption of H⁺ ions instead. The contact time of HCl solution with the as-received flyash particles is 1 h instead of 4 h. By this process, the HTN-flyash composite particles (non-magnetic) containing ˜2 wt. % H and 9 wt. % HTN (balance being the weight of flyash particles) are obtained.

The TEM image and EDX pattern of HTN-flyash composite particles, processed via the surface-sensitization of flyash using the H⁺ ions, are presented in FIGS. 18( a) and 18(b). The as-received flyash particles and the HTN-flyash composite particles are then utilized in the MB dye-adsorption experiments, conducted in the dark-condition, as described earlier in the Example-2 except that the initial MB dye concentration of 15 μM is used in this example. The variation in the normalized concentration of surface-adsorbed MB dye as a function of contact time, as obtained for the as-received flyash particles and micro-nano integrated HTN-flyash composite particles (with latter surface-sensitized with H⁺ ions), is presented and compared in FIG. 19. Relative to the equilibrium MB dye adsorption exhibited by the as-received flyash particles, the amount of MB dye adsorbed at equilibrium is seen to enhance due to the anchoring of HTN on the surface of flyash particles which are pre-sensitized with the H⁺ ions.

Example-9

In this example, the HTN-γ-Fe₂O₃ magnetic nanocomposite is synthesized via an innovative approach involving an ion-exchange mechanism operating under the dark-condition in an aqueous solution. As mentioned in the Example 7, Fe³⁺.cations can be used as the surface-sensitizer for the as-received flyash particles to anchor the HTN to their surfaces. Since the magnetic γ-Fe₂O₃ is the oxide of Fe³⁺ ions, the surface-sensitization step is not necessary and may be eliminated to anchor the HTN to the surface of γ-Fe₂O₃ via an ion-exchange mechanism operating under the dark-condition in an aqueous solution.

To demonstrate this, 0.5 g (or 0.1 g) of γ-Fe₂O₃ are dispersed in 125 ml of distilled-water having the neutral solution-pH (˜6.5-7.5) under the continuous overhead stirring at 25° C. for 10 min. 0.5 g (or 0.9 g) of hydrothermally processed HTN are then added to the above suspension under the continuous overhead stirring at 25° C. for 10 min. The resulting suspension is sonicated for 10 min and then stirred continuously under the dark-condition at 25° C. for 8 h using an overhead stirrer. The nano-nano integrated HTN-γ-Fe₂O₃ magnetic nanocomposite particles, thus formed, are separated from the aqueous solution using an external magnetic field, washed using 100 ml of distilled water for 1 h multiple-times till the pH of filtrate remains unchanged or neutral, again separated from the aqueous solution using an external magnetic field, and then dried in an oven at 80° C. for 12 h to obtain the HTN-γ-Fe₂O₃ magnetic nanocomposite particles containing 50 wt. % (or 10 wt. %) of γ-Fe₂O₃ particles (balance being the weight of HTN).

The TEM image of as-received nanocrystalline γ-Fe₂O₃ particles is shown in FIG. 20( a) and the corresponding EDX spectrum is presented in FIG. 20( b). The size of as-received nanocrystalline γ-Fe₂O₃ magnetic particles is within the range of 15-25 nm having near-spherical morphology. The EDX spectrum confirms the presence Fe and O within these nanoparticles. The selected-area electron diffraction (SAED) pattern; shown as an inset in FIG. 20( a), exhibits the concentric ring pattern indicating the nanocrystalline nature of as-received γ-Fe₂O₃ nanoparticles in agreement with the TEM image. The XRD pattern of as-received γ-Fe₂O₃ nanoparticles is presented in FIG. 20( c). The as-received magnetic nanoparticles possesses maghemite (γ-Fe₂O₃) structure as confirmed after comparing the XRD pattern with the JCDPS card no. 39-1346. The TEM images of HTN-γ-Fe₂O₃ magnetic nanocomposites are shown in FIG. 21, which clearly shows the attaching or anchoring of HTN at the short-edges (tube-openings) to the surface of γ-Fe₂O₃ magnetic nanoparticles via an ion-exchange mechanism operating under the dark-condition in an aqueous solution. It is noted that the magnetic nanoparticles are not dispersed along the surface of HTN which makes large number of potential-sites available for the dye-adsorption process, which could not be possible if the magnetic nanoparticles would have been dispersed along the surface of HTN. It appears that the attaching or anchoring of HTN typically at the short-edges (tube-openings) to the surface of γ-Fe₂O₃ magnetic nanoparticles via an ion-exchange mechanism takes place only at the neutral solution-pH. Similar process conducted in an acidic or basic range may lead to the formation of HTN-γ-Fe₂O₃ magnetic nanocomposites with the magnetic nanoparticles dispersed along the surface of HTN via the electrostatic attraction mechanism involving both the coulombic and van der Waals forces. The EDX pattern obtained using the HTN-γ-Fe₂O₃ magnetic nanocomposites, corresponding to FIG. 21( b), is shown in FIG. 22 which shows the presence of Ti and Fe as the major elements confirming the formation of nanocomposite particles. It is to be noted that the γ-Fe₂O₃ magnetic nanoparticles are not surface-sensitized for attaching or anchoring the HTN to their surfaces, which is however an essential step in the case of as-received flyash particles as described in the Example 1. In FIG. 23, the use of HTN-γ-Fe₂O₃ magnetic nanocomposite particles for the removal of MB dye from an aqueous solution, via the surface-adsorption process involving the ion-exchange and electrostatic-attraction mechanisms operating in the dark-condition has been demonstrated. The separation of HTN-γ-Fe₂O₃ magnetic nanocomposite particles, after the MB dye adsorption on the surface, using a moderate external magnetic field is clearly visible.

Thus, the processing of semiconductor-oxide nanotubes (HTN)-metal oxide (γ-Fe₂O₃) magnetic nanocomposite particles is successfully demonstrated, without the surface-sensitization step, via an innovative process involving an ion-exchange mechanism operating under the dark-condition in an aqueous solution.

Example-10

Similar to the Example 6, the previously adsorbed-dye can be decomposed on the surface of HTN-γ-Fe₂O₃ magnetic nanocomposite by mechanically mixing and suspending it with the non-magnetic nanocrystalline anatase-TiO₂ photocatalyst (˜3-30 nm average diameter) in an aqueous solution and exposing the resulting suspension to the UV or solar-radiation under the continuous overhead stirring till the adsorbed-dye is completely decomposed. Large concentration of OH. is produced by the nanocrystalline anatase-TiO₂ photocatalyst in shorter time relative to that produced by the HTN and γ-Fe₂O₃, FIG. 24, which attack and decompose the MB dye adsorbed on the surface of magnetic nanocomposite particles in relatively less time. The HTN-γ-Fe₂O₃ (5 wt. %) magnetic nanocomposite particles (1.0 g) are first prepared using the procedure described in the Example-9 except that the amount of γ-Fe₂O₃ and HTN used are changed to 0.05 g and 0.95 g. The magnetic nanocomposite particles are then suspended and stirred at 25° C. for 1 h in the dark-condition using an overhead stirrer in 125 ml aqueous solution of MB dye (30 μM). After the magnetic separation, the magnetic nanocomposite particles with the surface-adsorbed MB dye are dried in an oven at 80° C. for 12 h. Two different sets of samples are prepared. One set of HTN-γ-Fe₂O₃ (5 wt. %) magnetic nanocomposite particles with the surface-adsorbed MB dye are then used for the dye-adsorption experiment conducted with the initial MB dye concentration of 100 μM. The second set of HTN-γ-Fe₂O₃ (5 wt. %) magnetic nanocomposite particles with the surface-adsorbed MB dye are subjected to the surface-cleaning treatment. In this treatment, 1.0 g of HTN-γ-Fe₂O₃ (5 wt. %) magnetic nanocomposite particles with the surface-adsorbed MB dye (1.2 mg·g⁻¹) is suspended in 125 ml aqueous solution under the continuous overhead stirring. 1.0 g (50 wt. % of total amount of suspended solid particles) of non-magnetic nanocrystalline anatase-TiO₂ photocatalyst is then added. The resulting suspension is sonicated for 15 min and then stirred at 25° C. using an overhead stirrer under the UV-radiation exposure for 2 h. The mixture of surface-cleaned magnetic nanocomposite particles and the non-magnetic nanocrystalline anatase-TiO₂ photocatalyst are separated together using a centrifuge operated at 3000 rpm and washed using 100 ml of distilled-water for 1 h multiple-times till the pH of filtrate remains unchanged or neutral. The surface-cleaned magnetic nanocomposite particles are separated from the non-magnetic nanocrystalline anatase-TiO₂ photocatalyst using an external magnetic field. The separated powders are dried in an oven at 80° C. for 12 h. The surface-cleaned magnetic nanocomposite particles are recycled for the next-cycle of dye-adsorption conducted in the dark-condition using an aqueous solution containing 100 μM of MB dye.

The variation in the normalized concentration of surface-adsorbed MB dye as a function of contact time as obtained for the HTN-γ-Fe₂O₃ (5 wt. %) magnetic nanocomposite particles, before and after surface-cleaning treatment, is presented in FIG. 25. In 30 μM MB dye solution, the HTN-γ-Fe₂O₃ (5 wt. %) magnetic nanocomposite particles adsorb almost 100% of dye on the surface. If the MB dye adsorption is conducted, without the surface-cleaning treatment, using an aqueous solution containing 100 μM of MB dye, then the amount of dye-adsorbed on the surface of nanocomposite particles is noted be 50%. On the other hand, if the MB dye adsorption is conducted, after the surface-cleaning treatment, using an aqueous solution containing 100 μM of MB dye, then the amount of dye-adsorbed on the surface of nanocomposite particles is noted be 90%. The increase in the amount of dye-adsorption after the surface-cleaning treatment is, thus, clearly visible. This strongly suggests the successful decomposition of previously adsorbed-dye on the surface of HTN-γ-Fe₂O₃ (5 wt. %) magnetic nanocomposite particles by mixing them with the non-magnetic nanocrystalline anatase-TiO₂ photocatalyst in an aqueous solution and subjecting the suspension to the UV-radiation. It is obvious that any other non-magnetic photocatalyst, including CdS, ZnS, and other, may be selected for the surface-cleaning treatment instead of anatase-TiO₂.

It is to be noted that the pure nanocrystalline anatase-TiO₂ used for the surface-cleaning treatment get attached to (or anchored to) the HTN-γ-Fe₂O₃ magnetic nanocomposite particles via an ion-exchange mechanism operating the under the dark-condition in an aqueous solution. As a result, some amount of (non-magnetic) photocatalyst particles is lost after the magnetic separation. This has been observed to affect the amount of dye-adsorbed by the HTN-γ-Fe₂O₃ magnetic nanocomposite particles in the subsequent dye-adsorption cycles (2^(nd) cycle) following the surface-cleaning treatment. The latter issue becomes severe with the increasing number of dye-adsorption cycles. This is also particularly noted for the industry effluent sample containing a cold reactive dye (Red M5B). In order to avoid the interaction between the HTN-γ-Fe₂O₃ magnetic nanocomposite particles and the nanocrystalline anatase-TiO₂ photocatalyst particles, (that is to minimize the operation of an ion-exchange mechanism in between the two), the latter is required to be coated with the noble-metal selected from the group of Pt, Au, Pd, Ag, nanoparticles which can not only reduce the said interaction but also help enhance the concentration of OH′ produced under the radiation-exposure. This has been demonstrated in the following example using the above industry effluent sample received from the Rubmach Industries, Ahmedabad, Gujarat, India.

Example-11

In this example, Pt is first deposited on pure anatase-TiO₂ via the UV-reduction process. First, platinum(II) chloride (PtCl₂) is dissolved in a proper concentration range in 200 ml of distilled H₂O under the continuous magnetic stirring. 1 g of pure anatase-TiO₂ is then dispersed so as to obtain the Pt/Ti mole-ratio varying in the range of 1, 5, and 10%. The initial solution-pH is then adjusted to ˜10 by the addition of NH₄OH solution. The resulting suspension is then exposed to the UV-radiation in a Photoreactor (Luzchem, Canada) for 4 h. The Pt-deposited nanocrystalline anatase-TiO₂ is separated using a centrifuge operated at 3000 rpm and dried in an oven at 80° C. for 12 h. A typical TEM image of Pt-deposited nanocrystalline anatase-TiO₂ having the Pt/Ti mole-ratio of 5% is shown in FIG. 26( a) where very fine Pt nanoparticles dispersed on the surface of one of the nanocrystalline anatase-TiO₂ particle is clearly seen.

In order to prepare HTN-γ-Fe₂O₃ (10 wt. %) magnetic nanocomposite particles, the procedure already described in the Example-9 is followed. 0.3 g of HTN-γ-Fe₂O₃ (10 wt. %) magnetic nanocomposite particles are first suspended and stirred using an overhead stirrer in the dark-condition at 25° C. for 1 h in 125 ml aqueous solution containing 1 vol. % industry effluent sample. (NOTE: The industry effluent sample contains 10% of cold reactive Red M5B dye). The magnetic nanocomposite particles with the surface-adsorbed reactive-dye are separated using a magnetic separator and dried in an oven at 80° C. for 12 h. The second-cycle of dye-adsorption is conducted using the dried magnetic nanocomposite particles having the previously adsorbed reactive-dye on the surface. In another set of experiments, HTN-γ-Fe₂O₃ (10 wt. %) magnetic nanocomposite particles with the surface-adsorbed reactive-dye are prepared, which are subjected to the surface-cleaning treatment under the UV-radiation exposure using the Pt-deposited nanocrystalline anatase-TiO₂ having the varying Pt/Ti mole-ratio within the range of 1-10%. During the surface-cleaning treatment, 0.3 g of HTN-γ-Fe₂O₃ (10 wt. %) magnetic nanocomposite particles with the surface-adsorbed reactive-dye is suspended in 100 ml aqueous solution under the continuous overhead stirring at 25° C. for 10 min. 0.3 g (50 wt. % of total amount of suspended solid particles) of non-magnetic Pt-deposited nanocrystalline anatase-TiO₂ photocatalyst is then added under the continuous overhead stirring at 25° C. for 10 min. The resulting suspension is sonicated for 10 min and then stirred using an overhead stirrer under the UV-radiation exposure for 5 h. Due to the Pt-deposition, the HTN within the magnetic nanocomposite particles are not attached to (or anchored to) the nanocrystalline anatase-TiO₂ photocatalyst particles. The mixture of surface-cleaned magnetic nanocomposite particles and the non-magnetic nanocrystalline Pt-deposited anatase-TiO₂ photocatalyst are separated together using a centrifuge operated at 3000 rpm and washed using 100 ml of distilled-water for 1 h multiple-times till the pH of filtrate remains unchanged or neutral. The surface-cleaned magnetic nanocomposite particles are separated from the non-magnetic Pt-deposited nanocrystalline anatase-TiO₂ photocatalyst using an external magnetic field. The separated powders are dried in an oven at 80° C. for 12 h. The surface-cleaned magnetic nanocomposite particles are then recycled for the next-cycle of dye-adsorption conducted in the dark-condition. The amount of reactive-dye remaining in the solution, after each dye-adsorption experiments, is monitored via the UV-visible absorption spectrophotometer.

The variation in the UV-visible absorption intensity obtained after the dye-adsorption experiments conducted under the different conditions, as observed for the HTN-γ-Fe₂O₃ (10 wt. %) magnetic nanocomposite particles, is presented in FIG. 26( b). The cold reactive dye Red M5B exhibits the characteristic absorbance peak located at ˜500 nm. The variation in the intensity of this peak is monitored to judge the relative concentration of dye remaining in the aqueous solution after the dye-adsorption experiments. The intensity of absorbance-peak is higher for the initial dye-solution. The intensity however reduces significantly for the HTN-γ-Fe₂O₃ (10 wt. %) magnetic nanocomposite particles during the 1^(st) cycle of dye-adsorption. Nevertheless, when the HTN-γ-Fe₂O₃ (10 wt. %) magnetic nanocomposite particles with the previously adsorbed-dye is used for the 2^(nd) cycle of dye-adsorption, no significant dye-adsorption on the surface is noted. This strongly suggests that after the 1^(st) cycle of dye-adsorption, a surface-cleaning treatment is required to decompose the previously adsorbed-dye. Hence, the surface-cleaning treatment is conducted using the combination of Pt-deposited nanocrystalline anatase-TiO₂ having the Pt/Ti mole-ratio varying in the range of 1-10% and the UV-radiation exposure. It is observed that the dye-adsorption in the 2^(nd) cycle is enhanced considerably following the surface-cleaning treatment. It is further noted that the Pt-deposited nanocrystalline anatase-TiO₂ having the Pt/Ti mole-ratio of 10% exhibits the best performance during the surface-cleaning treatment. This has been attributed to both the significant reduction in the interaction between the HTN-γ-Fe₂O₃ (10 wt. %) magnetic nanocomposite particles and the nanocrystalline anatase-TiO₂ photocatalyst particles (that is, the minimization of operation of an ion-exchange mechanism in between the two) and enhanced concentration of OH. produced under the UV-radiation exposure due to the photo-induced electron-trapping effect of Pt. The pure nanocrystalline anatase-TiO₂ is not effective for the surface-cleaning treatment and increasing the dye-adsorption during the 2^(nd) cycle; hence, it is not included in FIG. 26( b) for clarity. It is obvious that the other noble-metals such as Au, Pd, Ag, and others may also work instead of Pt.

Example-12

In this example, the values of different parameters used for the processing of HTN-γ-Fe₂O₃-flyash magnetic composite particles are identical with those already described in the Example 1 except for the following changes. The initial SnCl₂ concentration is changed to 20 g·l⁻¹. The pure-HTN is replaced with HTN-γ-Fe₂O₃ (10 wt. %) magnetic nanocomposite which are processed using the method as already described in the Example 9. Hence, the final HTN-γ-Fe₂O₃-flyash magnetic composite particles contain ˜1 wt. % γ-Fe₂O₃ and ˜8 wt. % HTN (balance being the weight of Sn²⁺-sensitized flyash particles).

The TEM image and EDX spectrum obtained using the magnetic HTN-γ-Fe₂O₃-flyash composite particles, containing ˜1 wt. % γ-Fe₂O₃ and ˜8 wt. % HTN (balance being the weight of Sn²⁺-sensitized flyash particles), are presented in FIGS. 27( a) and 27(b). The comparison of FIG. 27( a) with FIG. 4( b) suggests the additional presence of magnetic γ-Fe₂O₃ nanoparticles on the surface of flyash particles which are pre-anchored to the HTN via the ion-exchange process operating under the dark-condition in an aqueous solution. This is further supported by the EDX analysis which shows (after the comparison with FIG. 3( a)) an increase in the relative intensities of Ti and Fe on the surface of as-received flyash particles. The magnetic γ-Fe₂O₃ nanoparticles provide the magnetic property; while, the HTN provide an enhanced dye-adsorption capacity to the composite particle. Thus, the processing of flyash-based magnetic dye-adsorbent composite is successfully demonstrated in this example.

The effect of vertically held magnet (M) on the as-received flyash particles and magnetic HTN-γ-Fe₂O₃-flyash composite particles, containing ˜1 wt. % γ-Fe₂O₃ and ˜8 wt. % HTN (balance being the weight of Sn²⁺-sensitized flyash particles), is compared in FIG. 28. As indicated by the arrows, the magnetic HTN-γ-Fe₂O₃-flyash composite particles strongly adhere to the vertically held magnet, FIG. 28( b), relative to that exhibited by the as-received flyash particles, FIG. 28( a). This strongly suggests that the HTN-γ-Fe₂O₃-flyash composite particles, containing ˜1 wt. % γ-Fe₂O₃ and ˜8 wt. % HTN (balance being the weight of Sn²⁺-sensitized flyash particles), can be separated from an aqueous solution using an external magnetic field which Is useful in the dye-removal application. Very weak magnetic property is also possessed by the as-received flyash particles possibly due to the presence of small amount of Fe₂O₃ in their structure, which is however not strong enough for their separation from an aqueous solution using an external magnetic field.

The as-received flyash particles and the HTN-γ-Fe₂O₃-flyash magnetic composite particles, containing ˜1 wt. % γ-Fe₂O₃ and ˜8 wt. % HTN (balance being the weight of Sn²⁺-sensitized flyash particles), are then utilized in the MB dye-adsorption experiments, conducted in the dark-condition, as described earlier in the Example-2 except that the initial MB dye concentration of 7.5 μM is used in this example. The variation in the normalized concentration of surface-adsorbed MB dye as a function of contact time, as obtained for the as-received flyash particles and HTN-γ-Fe₂O₃-flyash magnetic composite particles, containing ˜1 wt. % γ-Fe₂O₃ and ˜8 wt. % HTN (balance being the weight of Sn²⁺-sensitized flyash particles), is presented and compared in FIG. 29( a). Relative to the equilibrium MB dye adsorption exhibited by the as-received flyash particles, the amount of MB dye adsorbed at equilibrium is seen to be enhanced due to the anchoring of HTN-γ-Fe₂O₃ nanocomposite to the surface of flyash particles which are pre-sensitized with the Sn²⁺ cations. After the dye-adsorption process, the HTN-γ-Fe₂O₃-flyash magnetic composite particles could be separated from the treated aqueous solution using an external magnetic field, FIG. 29( b).

Thus, the processing of magnetic flyash-based composite particles, having an enhanced dye-adsorption capacity with the ability of magnetic separation, via an innovative approach is successfully demonstrated in this example. It is obvious that the magnetic flyash-based composite particles can be recycled in the dye-removal application using the technique similar to that used for the recycling of semiconductor-oxide nanotubes-metal oxide magnetic nanocomposite particles as demonstrated in the Example-11.

Example-13

In this example, the HTN-SnO composite particles are synthesized via an innovative approach involving an ion-exchange mechanism operating under the dark-condition in an aqueous solution. As demonstrated in the Example 1, Sn²⁺ cations can be used as the surface-sensitizer for the as-received flyash particles to anchor the HTN to their surfaces. Since SnO is the oxide of Sn²⁺ cations, the surface-sensitization step is not necessary and may be eliminated to anchor the HTN to the surface of SnO via an ion-exchange mechanism operating under the dark-condition in an aqueous solution.

To demonstrate this, 0.9 g of SnO particles (Sigma-Aldrich Chemicals, Bangalore, India) are dispersed in 100 nil of distilled-water under the continuous overhead stirring at 25° C. for 10 min. 0.1 g of hydrothermally processed HTN are then added to the above suspension under the continuous overhead stirring at 25° C. for 10 min. The resulting suspension is sonicated for 10 min and then stirred continuously under the dark-condition for 1 h using an overhead stirrer. The nano-nano integrated (10 wt. %) HTN-SnO composite particles, thus formed, are separated from the aqueous solution using a centrifuge operated at 3000 rpm, washed using 100 ml of distilled-water for 1 h multiple-times till the pH of filtrate remains unchanged or neutral, again separated from the aqueous solution via centrifuging, and then dried in an oven at 80° C. for 12 h to obtain the (10 wt. %) HTN-SnO composite particles.

The TEM image and EDX analysis of the (10 wt. %) HTN-SnO composite particles, processed via ion-exchange mechanism operating under the dark-condition in an aqueous solution, are presented in FIGS. 30( a) and 30(b) respectively. The corresponding SAED pattern is shown as an inset in FIG. 30( a). The anchoring of HTN at the shorter tube-edges (tube-openings) to the surface of SnO particle is clearly visible. Thus, the formation of (10 wt. %) HTN-SnO composite particles processed via an ion-exchange mechanism, operating under the dark-condition in an aqueous solution, without the involvement of surface-sensitization step is successfully demonstrated.

Advantages of the Invention

-   1. It provides the innovative semiconductor-oxide nanotubes-based     composite products (both magnetic and non-magnetic). -   2. It provides the innovative methods for the processing of     semiconductor-oxide nanotubes-based composite products (both     magnetic and non-magnetic). -   3. It provides a new industrial application to the innovative     semiconductor-oxide nanotubes-based composite products (both     magnetic and non-magnetic), typically the industrial dye-removal     from the aqueous solutions. -   4. It provides an innovative approach to increase the specific     surface-area of flyash without affecting its spherical morphology. -   5. It provides an innovative method to recycle the     semiconductor-oxide nanotubes-based products (both magnetic and     non-magnetic) used in the industrial dye-removal application. -   6. It provides an innovative solution for tackling the handling,     disposal, and recycling issues associated with the flyash without     and with the surface-adsorbed metal-cations. 

1. A composite particle which comprises: (i) metal-cations (M^(n+)) or protons-sensitized flyash particle of diameter in the range of 0.3-100 μm, (ii) nanotubes of semiconductor-oxides deposited on the surface of metal-cations or protons-sensitized flyash particle.
 2. The composite particle as claimed in claim 1, wherein the nanotubes are in the range of 1-15 wt. %, the meal-cations (M^(n+)) or protons in the range of 1-80 wt. %, and the balance being the weight of flyash particles.
 3. The composite particle as claimed in claim 1, wherein the flyash particle consists of the mixture of silica (SiO₂, 50-85 wt. %), alumina (Al₂O₃, 5-20 wt. %), iron oxide (Fe₂O₃, 5-15 wt. %), and trace amount of oxides of other elements selected from the group consisting of calcium, titanium and magnesium, and toxic heavy metals selected from the group consisting of arsenic, lead and cobalt.
 4. The composite particle as claimed in claim 1, wherein the metal-cations (M^(n+)) are selected from the group consisting of Sn²⁺/Sn⁴⁺, Fe²⁺/Fe³⁺, Pb²⁺, Zn²⁺, Cu²⁺, Mn²⁺, and Ti⁴⁺.
 5. The composite particle as claimed in claim 1, wherein the nanotubes of semiconductor-oxides are selected from the group consisting of hydrothermally processed hydrogen titanate (HTN, H₂Ti₃O₇ or the lepidocrocite-type), anatase-TiO₂ (ATN), and Ag-doped ATN.
 6. The composite particle as claimed in claim 1, wherein the nanotubes of semiconductor-oxides are optionally attached to the magnetic metal-oxide nanoparticles at the short-edges (tube-openings) and/or surface-deposited with the magnetic metal-oxide nanoparticles which are selected from the group consisting of γ-Fe₂O₃ Fe₃O₄, CoFe₂O₄, NiFe₂O, MnFe₂O₄, Co, Fe, and Ni.
 7. A process for the preparation of a composite particle as claimed in claim 1, wherein the said process comprises the steps of: (a) dissolving a metal-salt (1-70 g·l⁻¹) in water having an initial solution-pH of about 1-2 adjusted using the 0.1-1 HCl solution under continuous stifling for 30-300 min at temperature ranging between 20-30° C., followed by suspending flyash particles (1-25 g·l⁻¹) into it under continuous stirring for 30-300 min at temperature ranging between 20-30° C. to obtain the suspension of metal-cations (M^(n+)) sensitized flyash particles; (b) optionally, suspending flyash particles (1-25 g·l⁻¹) in 50-250 ml of water having the initial solution-pH of about 1-2 adjusted using the 0.1-1 M HCl solution under continuous stirring for 30-300 min at temperature ranging between 20-30° C. to obtain the suspension of protons-sensitized flyash particles; (c) adding hydrothermally processed nanotubes of semiconductor-oxides (0.1-5 g·l⁻¹) to the suspension of cations (M^(n+)) or protons-sensitized flyash particles as obtained in step (a) or in step (b), followed by sonicating the suspension for 5-30 min; (d) stirring the suspension obtained in step (c) continuously for 30-300 min at temperature ranging between 20-30° C. to obtain the composite of semiconductor-oxide nanotubes and cations (M^(n+)) or protons-sensitized flyash particles; and (e) separating the composite particles as obtained in step (d) using a centrifuge operated at 2000-4000 rpm or an external magnetic field, followed by washing the composite particles using water for 30 min-2h multiple-times until till the pH of filtrate remains unchanged or neutral, followed by separation and drying the composite particles in an oven at 70-90° C. for 10-15 h.
 8. The process as claimed in claim 7, wherein the metal-salt used in step (a) is selected from the group consisting of chloride, nitrate, sulfate-salts of Sn⁺/Sn⁺, Fe²7Fe³⁺, Pb²⁺, Zn²⁺, Cu²⁺, Mn²⁺, Ti⁺, and others.
 9. A nanocomposite particle which comprises: (i) metal-oxide nanoparticles in the range of 5-70 wt. %; (ii) nanotubes of semiconductor-oxides in the range of 30-95 wt. % attached to the surface of metal-oxide nanoparticles at the short-edges (tube-openings).
 10. The nanocomposite particle as claimed in claim 9, wherein the metal-oxide nanoparticles are selected from the group consisting of γ-Fe₂O₃ (magnetic), Fe₃O₄ (magnetic), SnO/SnO₂, PbO, ZnO, CuO, MnO, and TiO₂.
 11. The nanocomposite particle as claimed in claim 9, wherein the nanotubes of semiconductor-oxides are selected from the group consisting of hydrothermally processed hydrogen titanate (HTN, H₂Ti₃O₇ or the lepidocrocite-type) and anatase-TiO₂ (ATN).
 12. A process for the preparation of a nanocomposite particle as claimed in claim 9, wherein the said process comprises the steps of: (a) dispersing the 0.5-10 g·l⁻¹ of metal-oxide nanoparticles in water having a neutral solution-pH (of about 6.5-7.5) under continuous stifling for 5-30 min at temperature in the range of 20-30° C.; (b) adding 0.5-10 g·l⁻¹ of hydrothermally processed nanotubes of semiconductor-oxides in the suspension obtained in step (A) under continuous stifling for 5-30 min at temperature in the range of 20-30° C., followed by sonicating the suspension for 5-30 min, subsequently stifling the suspension continuously for 1-10 h in the dark-condition to obtain the semiconductor-oxide nanotubes-metal oxide nanocomposite particles; (c) separating the semiconductor-oxide nanotubes-metal oxide nanocomposite particles using a centrifuge operated at 2000-4000 rpm or an external magnetic field, followed by washing the nanocomposite particles using water for 30 min-2 h till the pH of filtrate remains unchanged or neutral, followed by separation and drying the nanocomposite particles in an oven at 70-90° C. for 10-15 h.
 13. The composite and nanocomposite particles as claimed in claim 1 are useful for the application involving the dye-removal from an aqueous solution and industry-effluent via the surface-adsorption mechanism operating in the dark-condition.
 14. A process for the surface-cleaning and the recycling of composite/nanocomposite particles as claimed in claim 1 after the adsorption of an organic synthetic-dye from an aqueous solution via the surface-adsorption mechanism operating in the dark-condition, comprising the steps of, a. suspending 1-30 g·l⁻¹ of composite/nanocomposite particles having the surface-adsorbed organic synthetic-dye (0.1-3 mg·g⁻¹) in water under continuous stirring for a period ranging between 5-30 min at temperature ranging between 20-30° C.; b. suspending a photocatalyst (10-60 wt. % of total weight of suspended solid particles) selected from the group of nanocrystalline anatase-TiO₂-coated SiO₂/γ-Fe₂O₃ magnetic photocatalyst or the noble-metal-deposited nanocrystalline anatase-TiO₂ in the suspension obtained in step (a) under continuous stirring, followed by sonicating the suspension for 5-30 min, subsequently stirring the suspension continuously under the UV or solar-radiation exposure for 1-10 h; c. centrifuging the solution at 2000-4000 rpm to separate the composite/nanocomposite particles and photocatalyst together, followed by washing using water for 30 min-2 h multiple-times until till the pH of filtrate remains unchanged or neutral; d. separating the photocatalyst particles from the surface-cleaned composite/nanocomposite particles using an external magnetic field; and e. drying both the photocatalyst particles and composite/nanocomposite particles in an oven at 70-90° C. for 10-15 h for the reuse.
 15. The composite and nanocomposite particles as claimed in claim 9 are useful for the application involving the dye-removal from an aqueous solution and industry-effluent via the surface-adsorption mechanism operating in the dark-condition.
 16. A process for the surface-cleaning and the recycling of composite/nanocomposite particles as claimed in claim 9 after the adsorption of an organic synthetic-dye from an aqueous solution via the surface-adsorption mechanism operating in the dark-condition, comprising the steps of, a. suspending 1-30 g·l⁻¹ of composite/nanocomposite particles having the surface-adsorbed organic synthetic-dye (0.1-3 mg·g⁻¹) in water under continuous stirring for a period ranging between 5-30 min at temperature ranging between 20-30° C.; b. suspending a photocatalyst (10-60 wt. % of total weight of suspended solid particles) selected from the group of nanocrystalline anatase-TiO₂-coated SiO₂/γ-Fe₂O₃ magnetic photocatalyst or the noble-metal-deposited nanocrystalline anatase-TiO₂ in the suspension obtained in step (a) under continuous stirring, followed by sonicating the suspension for 5-30 min, subsequently stirring the suspension continuously under the UV or solar-radiation exposure for 1-10 h; c. centrifuging the solution at 2000-4000 rpm to separate the composite/nanocomposite particles and photocatalyst together, followed by washing using water for 30 min-2 h multiple-times until the pH of filtrate remains unchanged or neutral; d. separating the photocatalyst particles from the surface-cleaned composite/nanocomposite particles using an external magnetic field; and e. drying both the photocatalyst particles and composite/nanocomposite particles in an oven at 70-90° C. for 10-15 h for the reuse. 